Micro corner cube array, method of making the micro corner cube array and reflective type display device

ABSTRACT

A method of making a micro corner cube array includes the steps of: preparing a substrate, at least a surface portion of which consists of cubic single crystals and which has a surface that is substantially parallel to {111} planes of the crystals; and etching the surface of the substrate anisotropically, thereby forming a plurality of unit elements for the micro corner cube array on the surface of the substrate. Each of these unit elements is made up of a number of crystal planes that have been formed at a lower etch rate than the {111} planes of the crystals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a micro corner cube array, a method ofmaking the micro corner cube array, and a reflective type display deviceincluding the micro corner cube array.

2. Description of the Related Art

In recent years, various types of optical elements having extremelysmall sizes (i.e., so-called “micro optical elements”), includingmicrolenses, micro mirrors and micro prisms, have been developed andmore and more extensively applied to the fields of opticalcommunications and display devices. It is expected that the opticaltechnology and display technology will be further developed and advancedby realizing those micro optical elements.

Examples of such optical elements include a corner cube reflector formedby arranging a plurality of corner cubes in a regular pattern. Each ofthose corner cubes has a shape corresponding to one corner of a cube andthree perpendicularly opposed reflective planes. The corner cubereflector is a type of retroreflector for reflecting an incoming lightray back to its source by getting the light ray reflected by each one ofthose reflective planes after another. The corner cube reflector canalways reflect the incoming light ray back to its source irrespective ofits angle of incidence. Hereinafter, conventional methods of making acorner cube will be described.

Plate Method

In a plate method, a number of flat plates, each having two mutuallyparallel planes, are stacked one upon the other. At the side end face ofthese flat plates stacked, V-grooves are cut vertically to the parallelplanes at an equal pitch, thereby forming a series of roof-shapedprotrusions each having an apical angle of approximately 90 degrees.Next, each of these flat plates is horizontally shifted with respect toadjacent one of them so that the tops of the series of roof-shapedprotrusions formed on the former plate are aligned with the bottoms ofthe V-grooves formed on the latter plate. In this manner, a die for useto make a corner cube array is obtained. In the plate method, a cornercube array is made by using this die. According to this method, however,it is necessary to accurately shift and secure the flat plate having theroof-shaped protrusions with respect to the adjacent flat plate so thatthese two plates satisfy a required positional relationship. Thus, it isdifficult to make a corner cube of as small a size as about 100 μm orless by this method.

Pin Bundling Method

In a pin bundling method, the end of a hexagonal columnar metal pin isprovided with a prism having three square facets that are opposedsubstantially perpendicularly to each other, and a number of such pinsare bundled together to make a collection of prisms. In this manner, acorner cube is made up of three facets of three prisms that are formedat the respective ends of three adjacent pins. According to this method,however, a corner cube should be made by collecting multiple prisms thathave been separately formed for mutually different pins. Thus, it isactually difficult to make a corner cube of a small size. The minimumpossible size of a corner cube that can be formed by this method isabout 1 mm.

Triangular Prism Method

In a triangular prism method, V-grooves are cut on the surface of a flatplate of a metal, for example, in three directions, thereby forming aplurality of triangular pyramidal protrusions and obtaining a collectionof prisms. However, the prisms formed by this method can have no othershape but the triangular pyramidal shape.

Furthermore, Japanese Laid-Open Publication No. 7-205322 discloses amethod of making a micro corner cube array by a photochemical technique.In this method, a mask having a plurality of equilateral triangulartransparent (or opaque) regions is used. Each of these transparent (oropaque) regions of this mask has variable transmittance (or opacity)that gradually decreases from its center toward its periphery. Byperforming exposing and developing process steps using such a mask, anumber of triangular pyramidal photoresist pattern elements are formedon a substrate. Then, the substrate, partially covered with thosephotoresist pattern elements, is etched anisotropically (e.g., dryetched) so as to have a plurality of protrusions in the same shape asthe photoresist pattern elements. In this manner, multiple triangularpyramidal protrusions, each having three isosceles triangular facetsthat are opposed substantially perpendicularly to each other, are formedon the substrate.

Furthermore, Japanese Laid-Open Publication No. 9-76245 discloses amethod of making a microlens array, which is characterized byselectively irradiating a predetermined region with a light beam.

In the fields of optical communications and display devices, those microoptical elements are often combined with a liquid crystal displaydevice, which needs to be relatively thin and lightweight as a colordisplay panel. As a color liquid crystal display device, a transmissivetype liquid crystal display device, including a backlight behind itsliquid crystal panel, is used more and more extensively in variousfields of applications. In the field of mobile communications units suchas cell phones on the other hand, a reflective type liquid crystaldisplay device for conducting a display operation by utilizing theambient light is used very often. Unlike the transmissive type liquidcrystal display device, the reflective type liquid crystal displaydevice needs no backlight, thus cutting down the overall powerdissipation and allowing the user to carry a downsized battery. For thatreason, the reflective type liquid crystal display device is not onlyeffectively applicable to various types of mobile electronic units thatshould be as lightweight and as thin as possible but also allows the useof a battery of an increased size when a unit including the reflectivetype display device is designed to have the same size and weight as aconventional one. This is because the space that has been left for abacklight can be used for other purposes in the reflective type displaydevice. Thus, the reflective type liquid crystal display device isexpected to increase the longest operating time of those units by leapsand bounds.

Also, an image displayed by a reflective type liquid crystal displaydevice has a better contrast than an image displayed by a display deviceof any other type even when the display device is used outdoors in thesun. For example, when a CRT, i.e., a self-light-emitting displaydevice, is used outdoors in the sun, the contrast ratio of an imagedisplayed thereon decreases considerably. Likewise, even a transmissivetype liquid crystal display device, subjected to low reflectiontreatment, also displays an image with a significantly decreasedcontrast ratio when the device is operated in an environment in whichthe ambient light is much intenser than the display light (e.g., indirect sunshine). On the other hand, a reflective type liquid crystaldisplay device increases the brightness of the image displayed thereonproportionally to the quantity of the ambient light, thus avoiding theunwanted decrease in contrast ratio. For that reason, a reflective typeliquid crystal display device is particularly suitable to mobileelectronic units that are often used outdoors, e.g., cell phones,notebook computers, digital cameras and camcorders.

Even though the reflective type liquid crystal display devices havethese advantageous features that are very useful in variousapplications, the reflective devices currently available are notentirely satisfactory yet in terms of its contrast ratio in dark place,definition, and full-color and moving picture display capabilities, forexample. Thus, the development of a more practically useful reflectivetype liquid crystal display device is awaited.

A reflective type liquid crystal display device used extensively todayoften includes one or two polarizers and typically operates in one ofthe following three modes:

-   -   Twisted nematic (TN) mode in which a display operation is        conducted by controlling the optical rotatory power of the        liquid crystal layer by an electric field;    -   Electrically controlled birefringence (ECB) mode in which a        display operation is conducted by controlling the birefringence        of the liquid crystal layer by an electric field; and    -   A mixed mode as a combination of the TN and ECB modes.

On the other hand, a guest host liquid crystal display device, in whicha dye is added to the liquid crystal material thereof, has been used asa device using no polarizers. However, a liquid crystal display deviceof this type is not so reliable because a dichroic dye has been added toits liquid crystal material, and cannot obtain a sufficiently highcontrast ratio because the dichroism ratio of the dye is low.Particularly when the contrast ratio is insufficient, a color displaydevice using color filters shows considerably decreased color purity. Inthat case, the display device needs to be combined with color filtersshowing high color purity. However, the use of color filters with highcolor purity results in decrease in lightness, thus losing theadvantages that are expected by the use of no polarizers.

In view of these considerations, a liquid crystal display device using apolymer-dispersed liquid crystal material or a cholesteric liquidcrystal material has been developed as a device that hopefully realizeshigh-lightness, high-contrast display without using any polarizer ordye. A liquid crystal display device of this type utilizes the uniqueproperties of the polymer-dispersed or cholesteric liquid crystal layer.Specifically, by controlling the voltage applied to the liquid crystallayer, the liquid crystal layer exhibits a transition between anoptically transmitting state and a scattering state or between thetransmitting state and a reflective state. A liquid crystal layerexhibiting any of these transitions will be herein referred to as a“scattering type liquid crystal layer” collectively. A liquid crystaldisplay device of this type uses no polarizers and can exhibit increasedoptical efficiency. In addition, even when a liquid crystal displaydevice of this type is evaluated in terms of color purity, a device ofthis type has lighter wavelength dependence than a device operating inthe TN or ECB mode. Also, when polarizers are used, the polarizers willhave a non-ideal absorption profile (i.e., the incident light becomesyellowish because a portion of the light falling within the bluewavelength range is absorbed into the polarizers). But a device of thistype has nothing to do with such a problem of the polarizers. Thus, thedevice is expected to realize a good display of color white.

A display device using a polymer-dispersed liquid crystal material isdisclosed in Japanese Laid-Open Publication No. 3-186816, for example.In the liquid crystal display device disclosed in that publication, apolymer-dispersed liquid crystal layer is provided over a blacksubstrate. When no voltage is applied to the polymer-dispersed liquidcrystal layer, the liquid crystal layer exhibits a scattering andnon-transparent state, thereby displaying color white. On the otherhand, when a voltage is applied to the polymer-dispersed liquid crystallayer, the liquid crystal layer exhibits a transmitting state and makesthe underlying black substrate visible to the observer, therebydisplaying color black.

In the liquid crystal display device disclosed in Japanese Laid-OpenPublication No. 3-186816, however, when color white is displayed, only aportion of the incident light that has been backscattered by thepolymer-dispersed liquid crystal layer contributes to the display ofcolor white but another portion of the light that has been forwardscattered is all absorbed into the black substrate. Accordingly, theoptical efficiency thereof actually decreases significantly.

A liquid crystal display device, including a light modulating layer madeof a light scattering liquid crystal material and a retroreflector, isdisclosed in U.S. Pat. Nos. 3,905,682 and 5,182,663, for example. In theliquid crystal display device disclosed in these patents, when colorblack should be displayed, the liquid crystal layer thereof iscontrolled in such a manner as to exhibit the transmitting state.Accordingly, in such state, the light that has been transmitted throughthe liquid crystal layer is reflected back to its source (i.e.,retro-reflected) by the retroreflector.

The corner cube reflector described above may be used as such aretroreflector. Hereinafter, a conventional liquid crystal displaydevice including a corner cube reflector will be described withreference to FIG. 40.

In the liquid crystal display device 900 shown in FIG. 40, a scatteringtype liquid crystal layer 6 is sandwiched between two transparentsubstrates 8 and 9, which are closer to, and more distant from, theobserver (not shown), respectively, and will be herein referred to as an“observer-side substrate” and a “non-observer-side substrate”,respectively. On one surface of the non-observer-side substrate 9 thatis opposed to the liquid crystal layer 6, a corner cube reflector 90 isprovided as a retroreflector. The reflective planes 90 a (i.e., therugged surface) of the corner cube reflector 90 are covered with atransparent flattening member 95 so as to be flattened. A transparentelectrode 12 is further formed on the transparent flattening member 95.On the other hand, on one surface of the observer-side substrate 8 thatis opposed to the liquid crystal layer 6, a color filter layer 7 andanother transparent electrode 12 are provided in this order.

By controlling a voltage applied to the pair of transparent electrodes12 that sandwiches the scattering type liquid crystal layer 6 betweenthem, the liquid crystal layer 6 can exhibit either a scattering stateor a transmitting state. When color white should be displayed, thescattering type liquid crystal layer 6 is controlled in such a manner asto enter the scattering state. In this mode, a portion of the incominglight, which has been incident from an external light source (e.g., thesun) onto the scattering type liquid crystal layer 6, is scattered bythe liquid crystal layer 6. Another portion of the incoming light isreflected from the corner cube reflector 90 and then scattered by theliquid crystal layer 6. On the other hand, when color black should bedisplayed, the scattering type liquid crystal layer 6 is controlled insuch a manner as to enter the transmitting state. In this mode, aportion of the incoming light that has been transmitted through thescattering type liquid crystal layer 6 is reflected back to its sourceby the corner cube reflector 90. Accordingly, just a portion of thelight that has been emitted from a light source near the observerreaches his or her eyes, thus realizing good display of color black. Inaddition, since the incoming light is retro-reflected, no regularlyreflected portion of the ambient light reaches the observer's eyes. As aresult, the unwanted back reflection of the surrounding sight isavoidable.

In the conventional reflective type liquid crystal display device 900including the corner cube reflector 90, the transparent electrode 12 andthe liquid crystal layer 6 are not in contact with the reflective planes90 a of the retroreflector 90 as shown in FIG. 40. That is to say, whenthe reflective planes 90 a of the retroreflector 90 are covered with theflattening member 95 as shown in FIG. 40, the incoming light may beabsorbed into the flattening member 95 or non-retro-reflected from theinterface between the flattening member 95 and the liquid crystal layer6. As a result, the desired high lightness may not be obtained or thecontrast ratio may decrease.

Also, in this liquid crystal display device 900, the size L1 of eachunit element of the corner cube reflector 90 needs to be equal to orsmaller than the size L2 of each picture element region. If the size L1of each unit element is greater than the size L2 of each picture elementregion, then a light ray, which has been transmitted through apredetermined picture element region and then retro-reflected from thecorner cube reflector, may pass through another picture element regionon the way back. In that case, the display may not be conducted asintended.

It should be noted that each of a predetermined number of elements,which is a constituent of one “pixel” as a minimum display area unit andwhich contributes to a display of its associated color, is hereinreferred to as a “picture element (or dot)”. In a full-color displaydevice, typically, one “pixel” is made up of three “picture elements”representing the three primary colors of red (R), green (G) and blue(B), respectively. Also, each region of the display device, which isprovided for the purpose of displaying the color represented by itsassociated picture element, will be herein referred to as a “pictureelement region”.

As described above, a corner cube for use in a liquid crystal displaydevice, for example, needs to have a very small size (e.g., about 100 μmor less). However, according to any of the above-described mechanicalmethods of making corner cubes, it is often difficult to make cornercubes of such a small size as intended due to some variations thatshould occur in an actual manufacturing process. Also, even if cornercubes of a very small size are made by one of the methods describedabove, then each reflective plane should have a low specularreflectivity and the radius R of curvature at each intersection betweentwo reflective planes should be great. As a result, the efficiency ofretro-reflection may decrease disadvantageously.

Also, as for a micro corner cube that has been made by a photochemicalmethod as disclosed in Japanese Laid-Open Publication No. 7-205322, itis difficult to ensure high plane precision (i.e., planarity) for eachside face (or reflective plane) thereof. In that method, the planeprecision of each side face of a micro corner cube depends on that of atriangular pyramidal photoresist pattern element formed on thesubstrate. However, to increase the plane precision of the photoresistpattern element, the processing steps of exposing and developing thephotoresist layer should be controlled strictly enough by making thevariation in transmittance or opacity of the mask constant, for example.Actually, though, such strict process control is hard to realize.Furthermore, according to this technique, each corner cube must be madeup of three rectangular isosceles triangular planes.

Also, the photochemical method of making a microlens array as disclosedin Japanese Laid-Open Publication No. 9-76245 uses a light beam.However, even if this technique is applied to making a micro corner cubearray, it is still difficult to ensure a sufficient plane precision (orplanarity) for each plane of a corner cube.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, an object of thepresent invention is to provide a method of making a micro corner cubearray of a very small size and with sufficiently high shape precision.

Another object of the present invention is to provide a display devicethat uses such a micro corner cube array.

Still another object of the present invention is to provide a reflectivetype display device that exhibits high lightness and high contrast ratioin displaying color white.

A method of making a micro corner cube array according to a preferredembodiment of the present invention includes the steps of: a) preparinga substrate, at least a surface portion of which consists of cubicsingle crystals and which has a surface that is substantially parallelto {111} planes of the crystals; and b) etching the surface of thesubstrate anisotropically, thereby forming a plurality of unit elementsfor the micro corner cube array on the surface of the substrate. Each ofthe unit elements is made up of a number of crystal planes that havebeen formed at a lower etch rate than the {111} planes of the crystals.

In one preferred embodiment of the present invention, the step b)preferably includes the step of forming {100} planes of the crystals atthe lower etch rate than the {111} planes thereof.

In this particular preferred embodiment, the step b) preferably includesthe step of forming the unit elements so that each said unit element ismade up of three {100} planes that are opposed substantiallyperpendicularly to each other.

Specifically, at least the surface portion of the substrate prepared inthe step a) may be made of a compound semiconductor having a sphaleritestructure.

In that case, the compound semiconductor is preferably gallium arsenideand the substrate preferably has a surface that is substantiallyparallel to {111}B planes formed by arsenic atoms.

Alternatively, at least the surface portion of the substrate prepared inthe step a) may be made of a material having a diamond structure.

In that case, at least the surface portion of the substrate preferablyconsists of germanium single crystals.

In another preferred embodiment of the present invention, the step b)preferably includes the step of etching the surface of the substrateanisotropically so that a ratio of the etch rate of the {111} planes tothe lower etch rate of the crystal planes is greater than 1.73.

In still another preferred embodiment, the method preferably furtherincludes, between the steps a) and b), the step c) of covering thesurface of the substrate with an etching mask layer. The etching masklayer preferably includes at least one masking element and at least oneopening that have been arranged to form a predetermined pattern.

In this particular preferred embodiment, the step b) preferably includesthe step of forming the unit elements for the micro corner cube array sothat the size of each said unit element is controlled in accordance withthe pattern of the etching mask layer defined in the step c).

Alternatively, the step c) may include the step of defining the etchingmask layer that includes a plurality of masking elements. Each of themasking elements preferably has a median point that is substantiallylocated at a honeycomb lattice point.

In that case, the masking elements are preferably spaced apart from eachother.

In still another preferred embodiment, the masking element preferablyhas a planar shape defined by at least three sides that are respectivelyparallel to (100), (010) and (001) planes of the crystals.

Specifically, the masking element preferably has a triangular planarshape defined by the three sides.

Alternatively, the masking element may have a planar shape defined by atleast three sides that are respectively parallel to (11-1), (1-11) and(−111) planes of the crystals.

In that case, the masking element preferably has a triangular planarshape defined by the three sides.

As another alternative, the masking element may have a planar shape thatis symmetrical about a three-fold rotation axis.

In that case, the masking element preferably has a hexagonal, nonagonalor dodecagonal planar shape.

In yet another preferred embodiment, the etching mask layer preferablyincludes a plurality of openings, each of which has a median point thatis substantially located at a honeycomb lattice point.

In yet another preferred embodiment, the at least one masking elementpreferably accounts for more than 50% of the total area of the etchingmask layer.

In yet another preferred embodiment, the step b) preferably includes thestep of stopping etching the surface of the substrate when a contactarea between the surface of the substrate and the masking element issubstantially minimized.

In yet another preferred embodiment, the step b) preferably includes thestep of subjecting the surface of the substrate to a wet etchingprocess.

In that case, the step b) preferably further includes the step ofsubjecting the surface of the substrate to a dry etching process atleast once.

In yet another preferred embodiment, the method preferably furtherincludes the step of transferring the shape of the unit elements, whichhave been formed on the surface of the substrate, to a resin material.

In yet another preferred embodiment, the step b) preferably includes thestep of forming the unit elements so that each said unit element is madeup of three substantially square planes that are opposed substantiallyperpendicularly to each other.

In yet another preferred embodiment, the surface of the substrateprepared in the step a) preferably defines an angle of greater than 0degrees and equal to or smaller than 10 degrees with the {111} planes ofthe crystals.

In this particular preferred embodiment, an intersection between thesurface of the substrate and the {111} planes of the crystals ispreferably substantially perpendicular to a cleaved facet of thesubstrate.

Another preferred embodiment of the present invention provides a methodof making an array of micro corner cubes, each being defined bypredetermined crystal planes of a crystal having a prescribed structure.The method includes the steps of: a) preparing a substrate, at least asurface portion of which consists of the crystals having the prescribedstructure; and b) etching the substrate anisotropically, therebyexposing the predetermined crystal planes intentionally.

Still another preferred embodiment of the present invention provides amicro corner cube array that has been made from a substrate, at least asurface portion of which consists of cubic single crystals. The microcorner cube array has unevenness including etched surfaces that havebeen formed from predetermined crystal planes of the crystals.

In one preferred embodiment of the present invention, the predeterminedcrystal planes are preferably {100} planes.

Yet another preferred embodiment of the present invention provides amold for a micro corner cube array. The mold has preferably been madefrom a substrate, at least a surface portion of which consists of cubicsingle crystals. The mold preferably has unevenness including etchedsurfaces that have been formed from predetermined crystal planes of thecrystals.

In one preferred embodiment of the present invention, the predeterminedcrystal planes are preferably {100} planes.

A display device according to yet another preferred embodiment of thepresent invention includes a micro corner cube array that has been madefrom a substrate, at least a surface portion of which consists of cubicsingle crystals. The array preferably has unevenness including etchedsurfaces that have been formed from predetermined crystal planes of thecrystals. The display device preferably further includes a lightmodulating layer, which is provided over the micro corner cube array.

In one preferred embodiment of the present invention, the micro cornercube array preferably includes a plurality of unit elements. Each of theunit elements preferably has a size smaller than a size of each pictureelement region of the display device.

Yet another preferred embodiment of the present invention provides amethod of making an array of micro corner cubes. The method preferablyincludes the steps of: preparing a base member on which a micro cornercube array pattern has been defined; and transferring the pattern onto amaterial for the micro corner cube array by using the base member as amold. In this method, when the material is stripped from the mold, anormal to one of multiple planes of the micro corner cube array patternand a direction in which the material is stripped from the mold arepreferably present in a single plane.

A reflective type display device according to yet another preferredembodiment of the present invention preferably includes: a substrate; aretroreflector; and a light modulating layer, which is interposedbetween the substrate and the retroreflector and which switches betweena scattering state and a transmitting state. The light modulating layeris preferably adjacent to reflective planes of the retroreflector.

In one preferred embodiment of the present invention, the lightmodulating layer is preferably a scattering type liquid crystal layer.

Specifically, in the transmitting state, the liquid crystal layerpreferably has liquid crystal molecules that are oriented continuouslyin both a thickness direction and in-plane directions of the liquidcrystal layer. The in-plane directions are perpendicular to thethickness direction.

Alternatively, in the transmitting state, the liquid crystal layerpreferably exhibits refractive index continuity with respect to lighttraveling in a thickness direction of the liquid crystal layer and tolight traveling in in-plane directions of the liquid crystal layer. Thein-plane directions are perpendicular to the thickness direction.

In this particular preferred embodiment, the liquid crystal layerpreferably includes: a first phase that does not respond to an electricfield externally applied; and a second phase that includes liquidcrystal molecules responding to the electric field. While the liquidcrystal layer is in the transmitting state, the first and second phasespreferably exhibit substantially the same refractive index with respectto both the light traveling in the thickness direction and the lighttraveling in the in-plane directions.

More specifically, the first phase preferably has a size of about 100 nmto about 20,000 nm.

In another preferred embodiment of the present invention, while theliquid crystal layer is in the transmitting state, a ratio of therefractive index that the first phase exhibits with respect to the lighttraveling in the thickness direction or the in-plane directions to therefractive index that the second phase exhibits with respect to the samelight is preferably from about 0.95 to about 1.05.

In still another preferred embodiment, the first phase preferablyincludes a polymer that has been obtained by polymerizing a monomerhaving a liquid crystal skeleton.

In yet another preferred embodiment, while no voltage is being appliedto the liquid crystal layer, the liquid crystal molecules, which arelocated over the reflective planes of the retroreflector or over thesubstrate, preferably have their major axes substantially aligned withthe thickness direction.

In this particular preferred embodiment, the liquid crystal moleculespreferably exhibit negative dielectric anisotropy.

In yet another preferred embodiment, the scattering state of thescattering type liquid crystal layer is preferably created by forming aplurality of liquid crystal domains, each having a predetermined size,in the liquid crystal layer. While the liquid crystal layer is in thetransmitting state, the liquid crystal molecules thereof are preferablyaligned substantially uniformly in the entire liquid crystal layer.

In this particular preferred embodiment, the predetermined size ispreferably about 100 nm to about 20,000 nm.

Alternatively, the scattering type liquid crystal layer preferablyincludes dispersion phases having a size smaller than the predeterminedsize. The liquid crystal domains are preferably formed due to disorderin orientation of the liquid crystal molecules that has been caused bythe dispersion phases.

In yet another preferred embodiment, the retroreflector preferablyincludes a plurality of retroreflecting elements, each of which hasthree reflective planes that are opposed substantially perpendicularlyto each other and which reflects incoming light back to its source Thelight modulating layer is preferably adjacent to the three reflectiveplanes.

In this particular preferred embodiment, the retroreflecting elementsare preferably arranged at a pitch of about 1 μm to about 1,000 μm.

In yet another preferred embodiment, the retroreflector preferablyincludes a micro corner cube array.

Other features, elements, processes, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of preferred embodiments of the presentinvention with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1F are cross-sectional views illustrating respectiveprocess steps for making a micro corner cube array according to a firstspecific preferred embodiment of the present invention.

FIGS. 2A through 2E illustrate some of the process steps for making themicro corner cube array of the first preferred embodiment:

FIG. 2A is a plan view of the structure shown in FIG. 1D;

FIGS. 2B and 2C are respectively a plan view and a perspective view ofthe structure shown in FIG. 1E; and

FIGS. 2D and 2E are respectively a plan view and a perspective view ofthe structure shown in FIG. 1F.

FIG. 3 is a plan view illustrating a photomask for use to make the microcorner cube array of the first preferred embodiment.

FIGS. 4A and 4B are plan views illustrating two other photomasks inrespective shapes that are different from that of the photomask shown inFIG. 3.

FIGS. 5A through 5I are cross-sectional views illustrating respectiveprocess steps for making a micro corner cube array according to a secondspecific preferred embodiment of the present invention.

FIGS. 6A through 6D illustrate some of the process steps for making themicro corner cube array of the second preferred embodiment:

FIGS. 6A and 6B are respectively a plan view and a perspective view ofthe structure shown in FIG. 5C or 5D; and

FIGS. 6C and 6D are respectively a plan view and a perspective view ofthe structure shown in FIG. 5E or 5F.

FIGS. 7A through 7D illustrate some of the process steps for making themicro corner cube array of the second preferred embodiment:

FIGS. 7A and 7B are respectively a plan view and a perspective view ofthe structure shown in FIG. 5G or 5H; and

FIGS. 7C and 7D are respectively a plan view and a perspective view ofthe structure shown in FIG. 5I.

FIGS. 8A through 8C are plan views illustrating three photomasks for useto make the micro corner cube array of the second preferred embodiment.

FIG. 9 is a cross-sectional view illustrating a configuration for areflective type liquid crystal display device according to a fifthspecific preferred embodiment of the present invention.

FIGS. 10A and 10B are cross-sectional views illustrating how colormixture may occur depending on the pitch of unit elements of a microcorner cube array in the reflective type liquid crystal display deviceof the fifth preferred embodiment:

FIG. 10A illustrates a situation where the pitch of unit elements of themicro corner cube array is greater than the size of each picture elementregion; and

FIG. 10B illustrates a situation where the pitch of unit elements of themicro corner cube array is smaller than the size of each picture elementregion.

FIG. 11A illustrates perspective views of a corner cube consisting ofthree rectangular isosceles triangular planes; and

FIGS. 11B and 11C are respectively a perspective view and plan viewsillustrating an array of such corner cubes.

FIG. 12A is a perspective view illustrating a corner cube consisting ofthree square planes; and

FIGS. 12B and 12C are respectively a perspective view and plan viewsillustrating an array of such corner cubes.

FIG. 13 is a cross-sectional view illustrating, as a comparativeexample, a configuration for a conventional reflective type liquidcrystal display device.

FIG. 14 illustrates an arrangement for an apparatus for measuring thereflectivity of a reflective type liquid crystal display device.

FIG. 15 is a cross-sectional view illustrating a configuration for anorganic EL display device according to a sixth specific preferredembodiment of the present invention.

FIG. 16 schematically illustrates the process of making a micro cornercube array by transferring a pattern from a mold onto a resin inaccordance with a seventh specific preferred embodiment of the presentinvention.

FIGS. 17A and 17B are respectively a perspective view and a plan viewillustrating a micro corner cube array obtained by transferring thepattern onto a resin in the seventh preferred embodiment.

FIG. 18 is a perspective view illustrating a substrate that has asurface tilted from {111} planes.

FIGS. 19A and 19B illustrate how the cross-sectional shape of a concaveportion changes with a (111)B/(100) etch selectivity in a third specificpreferred embodiment of the present invention:

FIG. 19A is a cross-sectional view illustrating two concave portionsthat have been formed by using mutually different etchants; and

FIG. 19B illustrates cross sections of three concave portions that havebeen formed at mutually different (111)B/(100) etch selectivities.

FIGS. 20A and 20B illustrate how the cross-sectional shape of a concaveportion changes with the (111)B/(100) etch selectivity in the thirdpreferred embodiment:

FIG. 20A is a cross-sectional view illustrating a situation where theetch selectivity is not so high; and

FIG. 20B is a cross-sectional view illustrating a situation where theetch selectivity is high enough.

FIG. 21 is a plan view illustrating an etching mask layer, includingtriangular masking elements, according to a fourth specific preferredembodiment of the present invention.

FIG. 22 is a plan view illustrating an etching mask layer, includingtriangular masking elements that face different directions from thoseillustrated in FIG. 21, according to the fourth preferred embodiment.

FIG. 23 is a plan view illustrating an etching mask layer, includinghexagonal masking elements that have a relatively great total area,according to the fourth preferred embodiment.

FIG. 24 is a plan view illustrating an etching mask layer, includinghexagonal masking elements that have a smaller total area than thoseshown in FIG. 23, according to the fourth preferred embodiment.

FIG. 25 is a plan view illustrating an etching mask layer, includinghexagonal masking elements that have an even smaller total area thanthose shown in FIG. 24, according to the fourth preferred embodiment.

FIG. 26 is a plan view illustrating an etching mask layer, includinghexagonal masking elements in a different shape from those shown in FIG.23, 24 or 25, according to the fourth preferred embodiment.

FIG. 27 is a plan view illustrating an etching mask layer, includingnonagonal masking elements, according to the fourth preferredembodiment.

FIG. 28 is a plan view illustrating an etching mask layer, includingdodecagonal masking elements, according to the fourth preferredembodiment.

FIG. 29 is a plan view illustrating an etching mask layer, includingsquare masking elements, according to the fourth preferred embodiment.

FIG. 30 is a plan view illustrating a micro corner cube array that hasbeen formed by using the mask layer shown in FIG. 21.

FIG. 31 is a plan view illustrating a micro corner cube array that hasbeen formed by using the mask layer shown in FIG. 22.

FIGS. 32A and 32B illustrate how the shape of concave portions changeswith the etch time in the fourth preferred embodiment:

FIG. 32A is a cross-sectional view illustrating a situation where theetching process was performed for an adequate amount of time (e.g.,three minutes); and

FIG. 32B is a cross-sectional view illustrating a situation where theetching process was performed for a longer time.

FIG. 33 illustrates the shape of the etching mask layer for use in thefourth preferred embodiment.

FIG. 34 is a cross-sectional view illustrating a part of the reflectivetype liquid crystal display device shown in FIG. 9 to a larger scale.

FIGS. 35A through 35E are cross-sectional views illustrating respectiveprocess steps for fabricating the reflective type liquid crystal displaydevice shown in FIGS. 9 and 34.

FIG. 36 is a cross-sectional view illustrating where the incoming andoutgoing light rays pass in the reflective type liquid crystal displaydevice shown in FIG. 34 while the liquid crystal layer thereof is in thetransmitting state and in the scattering state, respectively.

FIG. 37 is a cross-sectional view illustrating how the scattering typeliquid crystal layer switches its states in a reflective type liquidcrystal display device according to a first specific example of aneighth specific preferred embodiment of the present invention.

FIG. 38 is a cross-sectional view illustrating how the scattering typeliquid crystal layer switches its states in a reflective type liquidcrystal display device according to second through sixth specificexamples of the eighth preferred embodiment.

FIG. 39 is a perspective view illustrating a corner cube array for usein the reflective type liquid crystal display device of the eighthpreferred embodiment.

FIG. 40 is a cross-sectional view illustrating a configuration for aconventional reflective type liquid crystal display device including amicro corner cube array.

FIG. 41 is a plan view showing a (111)B plane of a GaAs crystal.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In a method of making a micro corner cube array according to a preferredembodiment of the present invention, a micro corner cube array is madeby using a single crystalline substrate consisting of cubic crystals(which will be herein sometimes referred to as a “cubic singlecrystalline substrate”). The cubic single crystalline substrate may bemade of a compound semiconductor having a sphalerite structure or amaterial having a diamond structure. More specifically, a cubic singlecrystalline substrate, having a surface that is disposed substantiallyparallelly to {111} planes of the crystals, is prepared and has thatsurface patterned by being subjected to an anisotropic etching process.

It should be noted that the “substrate having a surface substantiallyparallel to {111} planes of crystals” herein refers to not only asubstrate having a surface parallel to the {111} planes of the crystalsbut also a substrate having a surface that defines an angle of about 0degrees to about 10 degrees with the {111} planes of the crystals.

The method of the present invention is partly characterized bypatterning the surface of the substrate by an anisotropic etchingprocess so that the etch rate of one crystal plane is different fromthat of another. For example, if the substrate is made up of GaAscrystals having a sphalerite structure as shown in FIG. 41, the etchrate of the {111}B planes of the crystals (i.e., the {111} planes formedby arsenic) is relatively high, while the etch rate of the {100} planes(i.e., crystal planes including (100), (010) and (001) planes) thereofis relatively low. Accordingly, the anisotropic etching process advancesin such a manner as to leave the {100} planes of the crystals. As aresult, concavo-convex portions are defined on the surface of thesubstrate by multiple unit elements, each being made up of the {100}planes of the crystals. As used herein, each “unit element”, defined bya {100} family of planes, will also be referred to as a “concaveportion” because the element is formed by an anisotropic etchingprocess. Each of those unit elements that have been formed in thismanner has three perpendicularly opposed planes (e.g., (100), (010) and(001) planes), thus forming a corner cube.

In a corner cube array formed by such a method, the three reflectiveplanes of each corner cube are matched with the {100} crystallographicplanes of a cubic crystal and exhibit very high shape precision. Also,the three reflective planes that make up each corner cube have goodplanarity, and each corner or edge, at which two or three of thereflective planes intersect with each other, has sufficient sharpness.Furthermore, the corner cube array has a stereoscopic shape in whichmultiple unit elements or corner cubes are arranged in a regularlypattern. In this array, the respective vertices of the corner cubes arelocated at substantially the same level (or within substantially thesame plane). Thus, a corner cube array like this can be used effectivelyas a retroreflector for reflecting an incoming light ray back to itssource.

Also, the size of each unit element (i.e., each corner cube) of thecorner cube array formed by the method of the present invention may beseveral tens μm or less by adjusting the feature size of a photoresistpattern (or resist mask) used in the etching process. Accordingly, acorner cube array of a very small size, which is suitably applicable foruse as a retroreflector for a liquid crystal display device, forexample, can be obtained.

It should be noted that the “cubic single crystalline substrate” used inpreferred embodiments of the present invention includes a substrateobtained by forming a single crystal layer on a supporting base memberof an amorphous or polycrystalline material. Also, the substrate doesnot have to be a flat plate but may have any other stereoscopic shape aslong as the substrate has a flat surface.

Hereinafter, preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings, in which eachmember having substantially the same function is identified by the samereference numeral.

EMBODIMENT 1

FIGS. 1A through 1F illustrate respective process steps for making amicro corner cube array according to a first specific preferredembodiment of the present invention. In this preferred embodiment, asingle crystalline substrate of GaAs, which is an exemplary compoundsemiconductor having a sphalerite structure, is used as the cubic singlecrystalline substrate on which a micro corner cube array should beformed.

First, as shown in FIG. 1A, a substrate 1, which consists of GaAs singlecrystals and of which the surface is one of {111}B planes, is preparedand has that surface mirror-polished. It should be noted that {111}Aplanes are formed by gallium atoms, while the {111}B planes are formedby arsenic atoms. FIG. 41 shows one of the {111}B planes of GaAs singlecrystals as viewed from over the substrate 1. In FIG. 41, theconfiguration of the GaAs crystal unit cell is indicated by the one-dotchains.

Next, as shown in FIG. 1B, the surface of the substrate 1 is spin-coatedwith a positive photoresist layer 2 with a thickness of about 1 μm. Thephotoresist layer 2 may be made of OFPR-800 (produced by Tokyo OhkaKogyo Co., Ltd.), for example.

Subsequently, after the photoresist layer 2 has been pre-baked at about100° C. for approximately 30 minutes, a photomask 3 is disposed over thephotoresist layer 2 to expose the photoresist layer 2 to radiationthrough the mask 3 as shown in FIG. 1C. In this preferred embodiment, aphotomask 3 such as that shown in FIG. 3 may be used. As shown in FIG.3, in this photomask 3, equilateral triangular opaque regions andinverse equilateral triangular transmissive regions are alternatelyarranged in each of the three directions defined by the three sides ofthe triangles. The photomask 3 is disposed over the substrate 1 so thatone of the three sides of each equilateral triangular pattern element ofthe mask 3 is parallel to the <01-1> direction of the GaAs crystal asshown in FIG. 41. It should be noted that the negative sign preceding adirection index herein indicates that the direction index is negative.In this preferred embodiment, each equilateral triangular patternelement of the mask 3 has a length of about 10 μm each side.

Thereafter, the exposed photoresist layer 2 is developed as shown inFIG. 1D. As a developer, NMD-32.38% (produced by Tokyo Ohka Kogyo Co.,Ltd.) may be used. In this manner, a photoresist pattern 2′ (consistingof masking elements for the anisotropic etching process) is defined asshown in FIG. 2A. It should be noted that FIG. ID is a cross-sectionalview taken along the line ID—ID shown in FIG. 2A.

In this preferred embodiment, the size of corner cubes to be formed iscontrolled in accordance with the feature size of the photoresistpattern 2′ (or the pattern of the mask 3). More specifically, the sizeof the corner cubes to be formed becomes approximately equal to thepitch Pr of each opening R0 between two masking elements of thephotoresist pattern 2′. In this preferred embodiment, the pitch Pr isabout 10 μm. It should be noted that the photoresist pattern 2′ will beherein sometimes referred to as “masking elements” and that thosemasking elements and the openings R0 will be herein sometimes referredto as an “etching mask layer” collectively. That is to say, the “etchingmask layer” is a patterned photoresist layer 2.

The photoresist pattern 2′ is not limited to that shown in FIG. 2A.Alternatively, the photoresist pattern may be either the pattern shownin FIG. 4A in which the centers of three equilateral triangular openingsR0 are located at the three vertices of a virtual equilateral triangle Tor the pattern shown in FIG. 4B in which the centers of three squareopenings R0 are located at the three vertices of the virtual equilateraltriangle T. In any case, those three equilateral triangles or squaresmay be spaced apart from each other. No matter which of these resistpatterns is used, the size of the corner cubes to be formed iscontrolled to be approximately equal to the pitch Pr of the openings R0.It should be noted that each of these photoresist patterns is preferablydefined so that one side of the virtual triangle T is parallel to the<01-1> directions of the GaAs crystals.

Next, as shown in FIGS. 1E and 1F, the substrate 1 is wet-etched whilestirring an etchant with a magnet stirrer. In this preferred embodiment,the wet etching process may be carried out at a temperature of about 0°C. for approximately 30 minutes using a mixture of NH₄OH:H₂O₂:H₂O=4:1:20as the etchant.

When the substrate 1 is half-etched as shown in FIG. 1E (e.g., whenabout 15 minutes has passed since the etching process was started), thesubstrate 1 will have been etched deeply at its regions R1 that are notcovered with the photoresist pattern 2′ as shown in FIGS. 2B and 2C.Also, the substrate 1 will also be side-etched even at its regions R2that are covered with the photoresist pattern 2′. In this etchingprocess, the {100} planes of the GaAs single crystals, including the(100), (010) and (001) planes, are less easy to etch that the otherplanes thereof. Thus, the etching process advances anisotropically so asto form the {100} planes.

Thereafter, when the substrate 1 has been etched to the intended depthas shown in FIG. 1F, concavo-convex portions, consisting of the {100}planes S of the GaAs single crystals, will be formed as shown in FIG. 2Dand 2E. As a result, a micro corner cube array is obtained. It should benoted that the photoresist pattern 2′ will also have been stripped whenthe etching process reaches the stage shown in FIG. 1F.

As can be seen from FIG. 2E, the micro corner cube array obtained inthis manner has a stereoscopic shape in which multiple convex portions10 a and multiple concave portions 10 b are combined with each other.Also, each of its unit elements (i.e., each of the corner cubes of thearray) is made up of three substantially square planes that are opposedsubstantially perpendicularly to each other. As shown in FIG. 2D, eachunit element or corner cube has a substantially hexagonal shape as viewfrom over the substrate 1. In this manner, the corner cube formed by themethod of this preferred embodiment has a more complex shape than theconventional corner cube that is made up of three rectangular isoscelestriangles. However, the corner cube of this preferred embodiment has avery small size of about 10+ μm. In addition, the shape precision (e.g.,the planarity of each of the three substantially square planes) thereofis also very high.

When this micro corner cube array is used as a portion of aretroreflector, a thin film of a reflective material (e.g., aluminum ortin) may be deposited by an evaporation process, for example, to asubstantially uniform thickness (e.g., about 200 nm) over the ruggedsurface of the GaAs substrate. In this manner, a retroreflector,including three substantially square reflective planes that are opposedsubstantially perpendicularly to each other, can be obtained.

Alternatively, an electroformed mold may be made from this substratewith the rugged surface and then the surface shape (or unevenness) ofthe mold obtained in this manner may be transferred onto a resinmaterial to obtain a micro corner cube array of the resin.

In the preferred embodiment described above, the substrate 1 is made ofGaAs single crystals. Alternatively, the substrate 1 may also be made ofsingle crystals of any other compound having a sphalerite structure,e.g., InP, InAs, ZnS or GaP.

EMBODIMENT 2

FIGS. 5A through 5I illustrate respective process steps for making amicro corner cube array according to a second specific preferredembodiment of the present invention. In this preferred embodiment, asubstrate consisting of germanium single crystals having a diamondstructure is used as the cubic single crystalline substrate on which amicro corner cube array should be formed.

First, in the process step shown in FIG. 5A, a substrate 4, whichconsists of germanium single crystals and of which the surface isparallel to a (111) plane of the germanium crystals, is prepared and hasthat surface mirror-polished. Next, an SiO₂ layer 5 is deposited by aCVD process, for example, to a thickness of about 200 nm on themirror-polished surface of the substrate 4. Then, the surface of theSiO₂ layer 5 is spin-coated with a positive photoresist layer 2 with athickness of about 1 μm. The photoresist layer 2 may be made of OFPR-800(produced by Tokyo Ohka Kogyo Co., Ltd.), for example.

Subsequently, in the process step shown in FIG. 5B, after thephotoresist layer 2 has been pre-baked at about 100° C. forapproximately 30 minutes, a photomask 3 a, including a plurality ofequilateral triangular transmitting regions of a relatively small sizeas shown in FIG. 8A, is disposed over the substrate to expose thephotoresist layer 2 to radiation through the photomask 3 a and developit. In this manner, a first photoresist pattern 2 a is defined.Thereafter, the SiO₂ layer 5 is etched by using the first photoresistpattern 2 a as a mask, thereby forming a first SiO₂ mask 5 a having aplurality of openings in the same shape as the counterparts of thephotomask 3 a shown in FIG. 8A.

Next, in the process step shown in FIG. 5C, by using the firstphotoresist pattern 2 a and first SiO₂ mask 5 a as an etching mask, thesubstrate 4 is dry-etched. As a result, multiple concave portions C1 areformed in a predetermined region of the surface of the substrate 4 asshown in FIGS. 6A and 6B.

Subsequently, in the process step shown in FIG. 5D, the firstphotoresist pattern 2 a is exposed to the radiation and developed againby using the photomask 3 b shown in FIG. 8B having transmitting regionsthat are greater in area than those of the photomask 3 a shown in FIG.8A. In this manner, a second photoresist pattern 2 b is defined. Then,the first SiO₂ mask 5 a is further etched by using the secondphotoresist pattern 2 b as a mask, thereby forming a second SiO₂ mask 5b having openings in the same shape as the counterparts of the photomask3 b.

Thereafter, in the process step shown in FIG. 5E, the substrate 4 isfurther dry-etched by using the second photoresist pattern 2 b andsecond SiO₂ mask 5 b as an etching mask. As a result, two-steppedconcave portions C2, having two depth levels, are formed in thepredetermined region of the substrate 4 as shown in FIGS. 6C and 6D.Specifically, in each concave portion C2, the center region thereof isdeeper than the peripheral region thereof.

Next, in the process step shown in FIG. 5F, the second photoresistpattern 2 b is exposed to the radiation and developed again by using thephotomask 3 c shown in FIG. 8C having transmitting regions that are evengreater in area than those of the photomask 3 b shown in FIG. 8B. Inthis manner, a third photoresist pattern 2 c is defined. Then, thesecond SiO₂ mask 5 b is further etched by using the third photoresistpattern 2 c as a mask, thereby forming a third SiO₂ mask 5 c havingopenings in the same shape as the counterparts of the photomask 3 c.

Thereafter, in the process step shown in FIG. 5G, the substrate 4 isfurther dry-etched by using the third photoresist pattern 2 c and thirdSiO₂ mask 5 c as an etching mask. As a result, three-stepped concaveportions C3, having three depth levels, are formed in the predeterminedregion of the substrate 4 as shown in FIGS. 7A and 7B.

In each of the exposure process steps shown in FIGS. 5B, 5D and 5F, eachof the photomasks 3 a, 3 b and 3 c is disposed over the substrate sothat one of the three sides of the equilateral triangular maskingelements (or non-masking elements) thereof is parallel to the <01-1>directions of the germanium crystals. As in the first preferredembodiment described above, the patterns of the photomasks 3 a, 3 b and3 b each have a pitch Pr of about 10 μm in this preferred embodimentalso. Furthermore, in each of the dry-etching process steps shown inFIGS. 5C, 5E and 5G, a mixture of CF₄ and O₂ gases is used as an etchinggas.

Thereafter, the third photoresist pattern 2 c and the third SiO₂ mask 5c are stripped in the process step shown in FIG. 5H.

Finally, in the process step shown in FIG. 5I, the substrate 4 iswet-etched while being rocked manually. In this preferred embodiment,the wet etching process may be carried out at a temperature of about 0°C. for approximately 5 minutes using a mixture of HF:H₂O₂:H₂O=1:1:4 asthe etchant. In this wet etching process, the {100} planes (includingthe (100), (010) and (001) planes) of the germanium crystals are etchedat a lower rate than the other planes. Thus, the wet etching processadvances anisotropically so as to leave these {100} planes. As a result,multiple corner cubes, defined by the {100} planes of the germaniumcrystals, are formed on the surface of the substrate as shown in FIGS.7C and 7D.

As described above, according to this preferred embodiment, steppedconcavo-concave portions, similar in shape to corner cubes, are formedin advance on the surface of the substrate by dry-etching the surface anumber of times using multiple etching masks having mutually differentopening sizes, and then the substrate with the stepped portions iswet-etched, thereby forming corner cubes as defined by the {100} planesof the germanium crystals.

When the micro corner cube array obtained in this manner is used as aportion of a retroreflector, a thin film of a reflective material (e.g.,silver or aluminum) may be deposited by an evaporation process, forexample, to a substantially uniform thickness (e.g., about 200 nm) overthe rugged surface of the germanium substrate. In that case, the thinfilm needs to be deposited so as to come into tight contact with therugged surface. In this manner, a retroreflector, including threesubstantially square reflective planes that are opposed substantiallyperpendicularly to each other, can be obtained.

Optionally, an electroformed mold may be made from this substrate withthe rugged surface and then the surface shape (or unevenness) of themold obtained in this manner may be transferred onto a resin material toobtain a micro corner cube array of the resin.

EMBODIMENT 3

Hereinafter, a method of making a micro corner cube array according to athird specific preferred embodiment of the present invention will bedescribed. In this third preferred embodiment, a micro corner cube arrayis also formed on the surface of a single crystalline substrate bysubjecting the substrate to an anisotropic etching process as in thefirst and second preferred embodiments described above. However, in thispreferred embodiment, multiple different types of solutions are used asetchants for the anisotropic etching process.

As shown in FIG. 19A, a GaAs substrate 101 is used as the singlecrystalline substrate and is selectively covered with an etching mask(i.e., the masking elements of the etching mask layer) 102. Then, thesubstrate 101 is etched anisotropically though the mask 102, therebymaking a micro corner cube array.

In this preferred embodiment, to investigate exactly how the shape ofthe resultant micro corner cube array changes with the type of etchantused for the anisotropic etching process, a mixture of ammonia water,hydrogen peroxide water and water (NH₄OH:H₂O₂:H₂O=15:15:70) and amixture of concentrated sulfuric acid, hydrogen peroxide water and water(H₂SO₄:H₂O₂:H₂O=80:5:15) are used as etchants A and B, respectively. Byusing these etchants A and B, the anisotropic etching process isperformed at about 20° C. for approximately 3 minutes. In this preferredembodiment, the micro corner cube array is formed as in the firstpreferred embodiment except these etching conditions.

FIG. 19A shows the shapes of micro corner cubes a1 and a2 that have beenformed by using the etchants A and B, respectively. In FIG. 19A, themicro corner cubes a1 and a2 are illustrated as having been formed onthe same substrate for convenience sake. Actually, though, these microcorner cubes a1 and a2 are formed on two different substrates.

As can be seen from FIG. 19A, the micro corner cube a1 has a sharpenedbottom (i.e., defines a sharp angle at the bottom), while the microcorner cube a2 has a planar bottom.

These results show that the etchant A is preferred to the etchant B tomake a micro corner cube array in a desired shape. Hereinafter, it willbe described with reference to FIG. 19B how the etchant used changes theshape of micro corner cubes being made.

FIG. 19B illustrates the shapes of three micro corner cubes b1, b2 andb3 that have been formed in three situations where the ratio of the etchrate of a (111)B plane of the GaAs single crystalline substrate 101 tothat of a (100) plane thereof is greater than 1.73, equal to 1.73 andsmaller than 1.73, respectively. In FIG. 19B, the etch rate of the(111)B plane is represented by the length of the arrow 103, while theetch rate of the (100) plane is represented by the length of the arrow104. Thus, in the following description, these arrows 103 and 104 willalso be referred to as “etch rates 103 and 104” because these arrows 103and 104 are vectors representing the directions and magnitudes of theetching reaction. It should be noted that the “etch rate of a crystalplane” herein means how much (or how deep) the material (i.e., thesubstrate 101 in this case) is etched away per unit time in a directionperpendicular to the crystal plane.

As shown in FIG. 19B, in the corner cube b1, the etch rate 103 of the(111)B plane is much higher than the etch rate 104 of the (100) plane,and therefore, the resultant concave portion 101 a has a sharpenedbottom 101 b. In the corner cube b3 on the other hand, the etch rate 103of the (111)B plane is not so much higher than the etch rate 104 of the(100) plane, and the resultant concave portion 101 a has a planar bottom101 b. The corner cube b2 also has a planar bottom 101 b but the sizeand shape of the bottom 101 b are the same as those of an opening 105 ofthe etching mask 102.

Next, the ratios of these etch rates 103 and 104 will be consideredquantitatively. A normal to the (111)B plane and a normal to the (100)plane define an angle of about 54.7 degrees as shown in FIGS. 20A and20B. In this case, if the ratio of the etch rate 103 of the (111)B planeto the etch rate 104 of the (100) plane is equal to about 1.73, then theetching process advances while forming a bottom of approximately thesame size as the opening 105 of the etching mask layer. That is to say,the etching process advances while keeping the size of the bottomapproximately equal to that of the opening 105.

In contrast, if the ratio of the etch rate 103 of the (111)B plane tothe etch rate 104 of the (100) plane is greater than about 1.73, thenthe planar bottom 101 b of the concave portion 101 a gradually decreasesits size as the etching process advances as shown in FIG. 20B. As aresult, a micro corner cube having a sharpened bottom can be formed.When a retroreflector is formed by using a micro corner cube arrayconsisting of corner cubes with such a sharpened bottom, theretroreflector can reflect the incoming light back to its source withmore certainty.

As described above, to make a micro corner cube with a sharpened bottom,the ratio of the etch rate of the (111)B plane to that of the (100)plane (which will be herein referred to as a “(111)B/(100) etchselectivity”) needs to be greater than about 1.73. The (111)B/(100) etchselectivity is preferably equal to or greater than about 1.8, morepreferably equal to or greater than about 3.0.

Accordingly, to make a micro corner cube array with high shapeprecision, an etchant that realizes a (111)B/(100) etch selectivity ofat least greater than about 1.73 is preferably used. Examples of suchetchants include not only the etchant A described above but also amixture of sodium hydroxide, hydrogen peroxide water and water(NaOH:H₂O₂:H₂O=5 g:5 g:90 g).

In this manner, by appropriately controlling the etch selectivitybetween the two types of crystal planes, a micro corner cube array witheven higher shape precision can be formed. In the preferred embodimentdescribed above, a micro corner cube array is supposed to be formed soas to have {100} planes of cubic single crystals as its reflectiveplanes. To obtain such a micro corner cube array, just the anisotropicetching process needs to be performed appropriately as described abovebecause the {100} planes are easily exposed as intended by doing so. Inthe micro corner cube array obtained in this way, its planes (i.e., the{100} planes of cubic single crystals) have sufficiently high planeprecision (or planarity).

The first, second and third preferred embodiments described above relateto various methods of making a micro corner cube array by etching thesurface of a substrate anisotropically. Alternatively, a similar microcorner cube array may also be formed by a selective growth process ofcrystals, not the anisotropic etching process. In that case, if crystalsare grown in such a manner as to expose predetermined crystal planesintentionally, a micro corner cube array can also be formed with highshape precision.

EMBODIMENT 4

Hereinafter, a fourth specific preferred embodiment of the presentinvention will be described. The fourth preferred embodiment relates toa method of making a micro corner cube array by defining an etching masklayer 110, made up of masking elements 102 and openings 105, over a GaAssubstrate 101 and then etching the substrate 101 anisotropically asshown in FIG. 21, for example. In this preferred embodiment, to know howthe shape of the micro corner cube array to be formed changes with thepattern of the etching mask layer 110 consisting of the masking elements102 and openings 105, the etching process is carried out by usingvarious types of etching mask layers 110 shown in FIGS. 21 through 29.

It should be noted that each of those etching mask layers 110 to bedescribed below may be defined by patterning a photoresist film,deposited on the substrate 101, using a photomask as already describedfor the first preferred embodiment. In that case, portions of thephotoresist film that remain after the film has been patterned (i.e.,the photoresist pattern 2′) correspond to the masking elements 102 ofthe etching mask layer 110, while the other removed portions of thephotoresist film correspond to the openings 105 of the etching masklayer 110.

In this preferred embodiment, the unit elements (i.e., the maskingelements 102) of the etching mask layer 110 are regularly arranged inpredetermined directions. More specifically, in each of three directions106A, 106B and 106C, which cross each other to define an angle of about60 degrees between them in a plane parallel to the surface of thesubstrate as shown in FIG. 21, the masking elements 102 are arrangedregularly as the unit elements. In this case, the pitch 106, which isdefined as a distance between the median points or centers of twoadjacent masking elements 102, is about 13 μm in each of these threedirections 106A, 106B and 106C.

It should be noted that the “unit element” herein typically refers toeach of the masking elements 102 that have the same planar shape andthat are spaced apart from each other. However, each “unit element” doesnot have to consist of just one masking element 102. Also, the “multiplemasking elements” are not always spaced apart from each other but may bebarely connected together.

As described above, after the etching mask layer 110 in any of thosevarious patterns has been defined over the single crystalline substrate101, the substrate 101 is subjected to an anisotropic etching process.In this preferred embodiment, the etching process may be carried out atabout 20° C. for about 3 minutes or about 5 minutes using a mixture ofammonia water, hydrogen peroxide water and water(NH₄OH:H₂O₂:H₂O=15:15:70) as an etchant.

Hereinafter, each of those various patterns of the etching mask layer110 will be described with reference to FIGS. 21 through 29.

The etching mask layer 110 shown in FIG. 21 has the same pattern as thatused for the first preferred embodiment. Each of the masking elements102 of this mask layer 110 has a planar shape of an equilateral trianglethat has three sides parallel to (100), (010) and (001) planes of thesingle crystalline substrate 101. That is to say, the three sides ofeach masking element 102 are parallel to the (100), (010) and (001)planes.

Next, the shape of the masking elements 102 will be described in furtherdetail on the supposition that the a, b and c axes of crystals aredefined so that the [111] directions of the crystals are a normal of thesurface of the substrate, i.e., so that {111}B planes are defined in the[111] directions against the normal definition, as shown in FIG. 33. Inthe crystal structure shown in FIG. 33, the {111}B planes of the crystalcorrespond to the surface of the substrate, and the masking elements 102are defined on the {111}B planes. As used herein, an edge of the maskingelement 102 that is “parallel to the (100) plane” is indicated by theline segment a1 shown in FIG. 33. That is to say, if a vector A1 (alsoshown in FIG. 21), which crosses the edge (or the line segment a1) atright angles and is directed from the inside of the masking element 102toward the outside thereof, has a direction substantially correspondingto the [−211] directions of the crystal, then the edge is regarded asbeing “parallel to the (100) plane”. Also, an edge of the maskingelement 102 that is “parallel to the (010) plane” is indicated by theline segment a2 shown in FIG. 33. That is to say, if a vector A2 (alsoshown in FIG. 21), which crosses the edge (or the line segment a2) atright angles and is directed from the inside of the masking element 102toward the outside thereof, has a direction substantially correspondingto the [1-21] directions of the crystal, then the edge is regarded asbeing “parallel to the (010) plane”. Furthermore, an edge of the maskingelement 102 that is “parallel to the (001) plane” is indicated by theline segment a3 shown in FIG. 33. That is to say, if a vector A3 (alsoshown in FIG. 21), which crosses the edge (or the line segment a3) atright angles and is directed from the inside of the masking element 102toward the outside thereof, has a direction substantially correspondingto the [11-2] directions of the crystal, then the edge is regarded asbeing “parallel to the (001) plane”.

Referring back to FIG. 21, two adjacent ones of the masking elements 102slightly overlap with each other at an overlap portion 107. That is tosay, the total area of the masking elements 102 accounts for more than50% of that of the etching mask layer 110. In other words, the totalarea of the masking elements 102 is greater than that of the openings105.

The median point (or the center) of each masking element 102 issubstantially located at a honeycomb lattice point. As used herein, the“honeycomb lattice points” refer to the vertices and median points ofrespective rectangular hexagons when a predetermined plane is denselypacked with the hexagons of completely the same shape with no gaps leftbetween them. The “honeycomb lattice points” also correspond to theintersections between first and second groups of parallel lines that aredefined in a predetermined plane. In this case, the first group ofparallel lines extend in a first direction and are spaced apart fromeach other at regular intervals, while the second group of parallellines extend in a second direction so as to define an angle of 60degrees with the first group of parallel lines and are spaced apart fromeach other at the same regular intervals as the first group of parallellines.

The etching mask layer 110 shown in FIG. 22 has a pattern similar tothat of the first preferred embodiment or that shown in FIG. 21.However, the orientation of each triangular masking element 102 on thesurface of the single crystalline substrate 101 is different.Accordingly, the masking elements 102 and the openings 105 of the masklayer 110 shown in FIG. 22 form the negative of the mask layer 110 shownin FIG. 21 so to speak.

In the mask layer 110 shown in FIG. 22, each of the masking elements 102thereof has a planar shape of an equilateral triangle that has threeedges that are parallel to the (11-1), (1-11) and (−111) planes of thesingle crystalline substrate 101. That is to say, the three edges ofeach masking element 102 are parallel to the (11-1), (1-11) and (−111)planes.

As used herein, an edge of the masking element 102 that is “parallel tothe (11-1) plane” is indicated by the line segment b1 shown in FIG. 33.That is to say, if a vector B1 (also shown in FIG. 22), which crossesthe edge (or the line segment b1) at right angles and is directed fromthe inside of the masking element 102 toward the outside thereof, has adirection substantially corresponding to the [−1-12] directions of thecrystal, then the edge is regarded as being “parallel to the (11-1)plane”. Also, an edge of the masking element 102 that is “parallel tothe (1-11) plane” is indicated by the line segment b2 shown in FIG. 33.That is to say, if a vector B2 (also shown in FIG. 22), which crossesthe edge (or the line segment b2) at right angles and is directed fromthe inside of the masking element 102 toward the outside thereof, has adirection substantially corresponding to the [−12-1] directions of thecrystal, then the edge is regarded as being “parallel to the (1-11)plane”. Furthermore, an edge of the masking element 102 that is“parallel to the (−111) plane” is indicated by the line segment b3 shownin FIG. 33. That is to say, if a vector B3 (also shown in FIG. 22),which crosses the edge (or the line segment b3) at right angles and isdirected from the inside of the masking element 102 toward the outsidethereof, has a direction substantially corresponding to the [2-1-1]directions of the crystal, then the edge is regarded as being “parallelto the (−111) plane”.

Referring back to FIG. 22, two adjacent ones of the masking elements 102slightly overlap with each other at an overlap portion 107. That is tosay, the total area of the masking elements 102 accounts for more than50% of that of the etching mask layer 110. In other words, the totalarea of the masking elements 102 is greater than that of the openings105. Also, the median point (or the center) of each masking element 102is substantially located at a honeycomb lattice point.

In the etching mask layers 110 shown in FIGS. 23, 24 and 25, the maskingelements 102 thereof have a rectangular hexagonal planar shape and arespaced apart from each other. Also, in the examples shown in FIGS. 23,24 and 25, the total area of the masking elements 102 respectivelyaccounts for about 75% , about 60% and about 50% of that of the etchingmask layer 110. Furthermore, in each of the etching mask layers 110shown in FIGS. 23, 24 and 25, the median point (or the center) of eachmasking element 102 is substantially located at a honeycomb latticepoint.

The total area of the mask layer 110 is equal to the sum of the totalarea of the masking elements 102 and that of the opening(s) 105. Moreexactly, the “total area of the mask layer 110” herein means the sum ofthe total areas of the masking elements 102 and opening(s) 105 in aregion where the masking elements 102 and opening(s) 105 form a regularpattern. Also, the ratio in total area of the masking elements 102 tothe etching mask layer 110 may be represented as a ratio of the totalarea of the masking elements 102 to the area of a rectangular region(e.g., as indicated by the dashed lines 106D in FIG. 23) obtained byconnecting together the respective centers of four adjacent maskingelements 102.

In the mask layer 110 shown in FIG. 26, each of the masking elements 102thereof has a planar shape of a rectangular hexagon that has six edgesthat are parallel to the (100), (010), (001), (11-1), (1-11) and (−111)planes of the single crystalline substrate 101. The masking elements 102are spaced apart from each other. Also, the median point (or the center)of each masking element 102 is substantially located at a honeycomblattice point. It should be noted that the rectangular hexagonal maskingelements 102 shown in FIG. 26 are similar to those obtained by rotatingthe rectangular hexagonal masking elements 102 shown in FIG. 23, 24 or25 by 90 degrees in a plane parallel to the surface of the substrate.Furthermore, in the example shown in FIG. 26, the total area of themasking elements 102 accounts for about 60% of that of the etching masklayer 110.

In the mask layer 110 shown in FIG. 27, each of the masking elements 102thereof has a planar shape of a nonagon that includes edges that areparallel to the (100), (010) and (001) planes of the single crystallinesubstrate 101. The masking elements 102 are spaced apart from eachother. Also, the median point (or the center) of each masking element102 is substantially located at a honeycomb lattice point.

In the mask layer 110 shown in FIG. 28, each of the masking elements 102thereof has a planar shape of a dodecagon that includes edges that areparallel to the (100), (010), (001), (11-1), (1-11) and (−111) planes ofthe single crystalline substrate 101. The masking elements 102 arespaced apart from each other. Also, the median point (or the center) ofeach masking element 102 is substantially located at a honeycomb latticepoint.

In the mask layer 110 shown in FIG. 29, each of the masking elements 102thereof has a square planar shape. The median point (or the center) ofeach masking element 102 is also substantially located at a honeycomblattice point.

When the substrate 101 was etched anisotropically by using the masklayers 110 having the respective patterns shown in FIGS. 21 through 29,the following results were obtained.

No matter which of the etching mask layers 110 shown in FIGS. 21 through29 is used, each vertex of convex portions (i.e., the highest-levelpoint) of a micro corner cube array is formed at the center of itsassociated masking element 102. As described above, the center of eachmasking element 102 in each of these mask layer 110 is substantiallylocated at a honeycomb lattice point. Thus, each of the highest-levelpoints of the micro corner cube array is formed on the honeycomb latticepoint. Accordingly, it can be seen that the center of each maskingelement 102 in each mask layer 110 preferably corresponds to a vertex ofa micro corner cube array to be formed (i.e., a honeycomb latticepoint). Also, the size of the micro corner cubes is controlled inaccordance with the pitch 106 defined as a distance between the centersof two adjacent masking elements 102. Consequently, it can be seen thatif an appropriate pattern is selected for the etching mask layer 100,then micro corner cubes of a desired size can be obtained.

FIGS. 30 and 31 illustrate two micro corner cube arrays to be formed byusing the etching mask layers 110 shown in FIGS. 21 and 22,respectively. In FIGS. 30 and 31, the open circles ◯ indicate thehighest-level points (i.e., the vertices of convex portions) of themicro corner cubes, the solid circles ● indicate the lowest-level points(i.e., the vertices of concave portions) of the micro corner cubes, andthe triangles Δ indicate intermediate-level points. As can be seen fromFIGS. 30 and 31, no matter which of the etching mask layers 110 shown inFIGS. 21 and 22 is used, the highest-level point ◯ of each micro cornercube is located at the median point of its associated masking element102. However, each micro corner cube is made up of the (100), (010) and(001) planes. Thus, when the mask layer 110 shown in FIG. 21 is used,the center of each opening 105 thereof corresponds to the lowest-levelpoint ● of its associated corner cube. On the other hand, when the masklayer 110 shown in FIG. 22 is used, the center of each opening 105thereof corresponds to an intermediate-level point Δ of its associatedcorner cube.

Also, no matter which of the etching mask layers 110 is used, if theetching process is performed for about 3 minutes, each vertex of theresultant micro corner cube array is sharpened as shown in FIG. 32A.However, if the etching process is performed for about 5 minutes, eachvertex of the resultant micro corner cube array is rounded as shown inFIG. 32B. The reason is as follows. As the etching process advances, thecontact area between the {111}B planes of the GaAs single crystallinesubstrate 101 (i.e., the surface of the substrate) and the maskingelements 102 decreases gradually. And when the masking elements 102 areno longer in contact with the surface of the substrate, the {111}Bplanes start to be etched from the vertices. As a result, the verticesof the corner cubes are rounded.

Thus, the present inventors discovered that the etching process shouldpreferably be stopped when the masking elements 102 are no longer incontact with the {111}B planes of the GaAs single crystalline substrate101 (i.e., when the contact area between them is minimized). To stop theetching process when the contact area between the {111}B planes and themasking elements is minimized, the optimum etch time needs to beobtained beforehand via experiments, for example. If the etch time isoptimized in this manner, then a micro corner cube array having thedesired sharpened vertices can be obtained.

The present inventors also discovered that depending on the type of theetchant used, sometimes it is preferable to reduce the area of theopenings 105 of the etching mask layer 110. This is because if the ratioof the etch rate of the {111}B planes to that of the {100} planes (i.e.,the (111) B/(100) etch selectivity described above) is not sufficientlyhigh, then the etching process may advance so as to leave a planarbottom inside each of the openings 105. That is to say, to obtain amicro corner cube array in a desired shape, the greater the area of eachopening 105 of the etching mask layer 110, the higher the etchselectivity should be. In other words, the smaller the area of eachopening 105 of the etching mask layer 110, the lower the etchselectivity may be. Accordingly, if the area of each opening 105 of theetching mask layer 110 is small, the etching condition may be lessstrict.

For example, let us consider three situations where the etching masklayers 110 shown in FIGS. 23, 24 and 25 were respectively used.Specifically, when the etching mask layer 110 shown in FIG. 23 or 24, inwhich the total area of the masking elements 102 accounted for more than50% of the total, was used, a micro corner cube array having a desiredshape could be obtained. However, when the etching mask layer 110 shownin FIG. 25, in which the total area of the masking elements 102accounted for approximately 50% of the total, was used, a micro cornercube array having a desired shape could not be obtained but had planarbottoms. Thus, the present inventors discovered that the total area ofthe masking elements 102 preferably accounts for more than 50% of theentire etching mask layer 110, i.e., the total area of the maskingelements 102 is preferably greater than that of the openings 105.

Next, it will be described how the shapes of the resultant micro cornercube arrays were different between the two situations where the etchingmask layers 110 shown in FIGS. 21 and 29 were respectively used. Thepresent inventors discovered that a good micro corner cube array couldbe formed no matter which of these mask layers 110 was used. However,when the present inventors estimated the plane precisions (or theplanarities) of corner cubes in these two types of arrays by electronmicroscopy, we discovered that the array obtained by using the masklayer 110 shown in FIG. 21 showed higher plane precision than the arrayobtained by using the mask layer 110 shown in FIG. 29. The micro cornercube array to be formed has a planar shape that is symmetrical about athree-fold rotation axis. Accordingly, each of the masking elements 102(or openings 105) of the mask layer 110 to be used also preferably has aplanar shape that is symmetrical about a three-fold rotation axis.

Next, it will be described how the shapes of the resultant micro cornercube arrays were different between the two situations where the etchingmask layers 110 shown in FIGS. 21 and 22 were respectively used. Thepresent inventors discovered that a good micro corner cube array couldbe formed no matter which of these mask layers 110 was used. However,when the present inventors estimated the plane precisions (or theplanarities) of corner cubes in these two types of arrays by electronmicroscopy, we discovered that the array obtained by using the masklayer 110 shown in FIG. 21 showed higher plane precision than the arrayobtained by using the mask layer 110 shown in FIG. 22. However, we alsodiscovered that when the mask layer 110 shown in FIG. 21 was used,unnecessary vertices (of convex portions) were formed here and there onthe surface of the substrate under the overlap portions 107 between themasking elements 102. Accordingly, to make a large number of cornercubes of a desired shape uniformly over a wide area, when the etchingmask layer 110 having the pattern shown in FIG. 21 is used, the masklayer 110 should have a higher shape precision compared to using theetching mask layer 110 having the pattern shown in FIG. 22.

On the other hand, when the mask layers 110 shown in FIGS. 23 through29, having no overlap portions 107 between the masking elements 102,were used, no unnecessary convex portions were formed on the surface ofthe substrate. Thus, it was discovered that those unnecessary convexportions were formed due to the existence of the overlap portions 107between the masking elements 102. In other words, the present inventorsdiscovered that the masking elements 102 are preferably spaced apartfrom each other in the etching mask layer 110.

As described above, by selecting an etching mask layer having anappropriate pattern for the etching process, the shape precision of theresultant micro corner cube array can be increased. Also, by definingthe median point of each masking element in the etching mask layerappropriately, the vertex locations and the size of the resultant microcorner cube array can be determined just as intended.

EMBODIMENT 5

Hereinafter, a fifth specific preferred embodiment of the presentinvention will be described. The fifth preferred embodiment relates to areflective type liquid crystal display device that uses, as aretroreflector, a micro corner cube array that has been made by any ofthe methods described for the first through fourth preferred embodimentsof the present invention.

FIG. 9 illustrates a configuration for a reflective type liquid crystaldisplay device 100 according to the fifth preferred embodiment. As shownin FIG. 9, this liquid crystal display device 100 includes a pair ofsubstrates 8 and 9 and a scattering type liquid crystal layer 6, whichis provided as a light modulating layer between the substrates 8 and 9.The substrate 8 is located closer to the observer, while the substrate 9is provided so as to face the substrate 8. These two substrates 8 and 9are both made of a transparent material and may be glass plates orpolymer films, for example.

On one surface of the substrate 8 that is opposed to the liquid crystallayer 6, a color filter layer 7, including R, G and B color filters, anda transparent electrode 12 are stacked in this order. On the other hand,on one surface of the substrate 9 that is opposed to the liquid crystallayer 6, a micro corner cube array 10 is provided. This micro cornercube array 10 is covered with a reflective electrode 11, which is madeof a material with a high surface reflectivity (e.g., silver oraluminum) and has a substantially uniform thickness. The reflectiveelectrode 11 may be formed so as to come into tight contact with therugged surface of the micro corner cube array 10 by depositing silver toa thickness of about 200 nm by an evaporation process, for example. Thereflective electrode 11 serves not only as reflective planes forreflecting the incoming light back to its source but also as anelectrode for applying a voltage to the liquid crystal layer 6.

The liquid crystal display device 100 having such a configurationdisplays an image thereon by controlling the light modulating states ofthe liquid crystal layer 6 on a pixel-by-pixel basis with a voltageapplied from the transparent electrode 12 and reflective electrode 11 tothe liquid crystal layer 6. The voltage applied to the electrodes 11 and12 may be controlled either by known active components such as thin-filmtransistors or by any other device.

In the preferred embodiment illustrated in FIG. 9, the micro corner cubearray 10 is formed on the substrate 9. Alternatively, the micro cornercube array 10 itself may be used as a substrate without providing thesubstrate 9. As already described for the first preferred embodiment,the micro corner cube array 10 may be formed of a GaAs substrate. When aGaAs substrate is used, a circuit for driving the active components andother circuits may be formed on the same substrate around the displayarea thereof, for example. If the driver circuit and other circuits canbe formed on the same substrate, then the overall size of the displaydevice can be reduced. Thus, such a display device is effectivelyapplicable to a cell phone, for example.

In this preferred embodiment, the scattering type liquid crystal layer 6is made of a polymer-dispersed liquid crystal material. However, thematerial of the liquid crystal layer 6 is not limited thereto, but mayalso be any other scattering type liquid crystal material such as anematic-cholesteric phase change type liquid crystal material or aliquid crystal gel. Furthermore, the liquid crystal layer 6 may also bemade of any of various other liquid crystal materials as long as thematerial allows the liquid crystal layer 6 to switch between a mode totransmit the incoming light and a mode to scatter the light at least.Specifically, examples of other usable liquid crystal materials include:a cholesteric liquid crystal material, which can switch betweentransmitting and reflecting states and to which diffusion properties areimparted by controlling the domain sizes of liquid crystal molecules;and a polymer-dispersed liquid crystal material with a holographicfunction, which switches between transmitting and reflecting states andto which diffusion properties are imparted by exposing the material todiffusing radiation.

The polymer-dispersed liquid crystal material used in this preferredembodiment is obtained by preparing a mixture of a low-molecular-weightliquid crystal composition and an unpolymerized prepolymer in a misciblestate, injecting the mixture into the gap between the substrates andthen polymerizing the prepolymer. Any type of polymer-dispersed liquidcrystal material may be used so long as the material is obtained bypolymerizing a prepolymer. In this preferred embodiment, a mixture of aUV-curing prepolymer with liquid crystal properties and a liquid crystalcomposition is photo-cured by being exposed to an active ray such as anultraviolet ray, and the cured mixture (i.e., a UV-cured liquid crystalmaterial) is used as the polymer-dispersed liquid crystal material. Ifsuch a UV-cured liquid crystal material is used as the polymer-dispersedliquid crystal material, the polymerizable liquid crystal material maybe polymerized without being heated. Thus, other members of the liquidcrystal display device are not affected by the heat generated.

The prepolymer-liquid crystal mixture may be obtained by adding a smallamount of polymerization initiator (produced by Ciba-Geigy Corporation,for example) to a mixture of a UV-curing material and a liquid crystalcomposition that have been mixed at a weight ratio of about 20:80, forexample. The prepolymer-liquid crystal mixture obtained in this mannerexhibits a nematic liquid crystal phase at room temperature. On enteringa liquid crystal layer made of such a material, the incoming light ismodulated in accordance with the mode (i.e., scattering or transmittingmode) of the liquid crystal layer that changes with the voltage appliedthereto. In this preferred embodiment, the liquid crystal layer exhibitsthe scattering state when no voltage is applied thereto and thetransmitting state when a voltage is applied thereto.

Hereinafter, it will be described specifically how the reflective typeliquid crystal display device 100 operates.

First, the operation of the display device 100 in a white display modewill be described. In the white display mode, the liquid crystal layer 6is controlled to exhibit the scattering state. Thus, externally incominglight is scattered by the liquid crystal layer 6 after having beentransmitted through the substrate 8 and the color filter layer 7. Inthis case, portion of the incoming light, which has been backscatteredby the liquid crystal layer 6, returns to the observer. Also, in thedisplay device of this preferred embodiment, another portion of theincoming light that has been transmitted straight through the liquidcrystal layer 6 and still another portion of the incoming light that hasbeen forward scattered by the liquid crystal layer 6 are reflected bythe reflective electrode 11 on the micro corner cube array 10. Thereflected portions of the light are scattered again by the liquidcrystal layer 6 in the scattering state while passing through the liquidcrystal layer 6. As a result, a portion of the scattered light returnsto the observer. In this manner, in the white display mode, not only thebackscattered light but also a portion of the forward scattered lightreturn to the observer, thus realizing a display of a highly brightimage.

Next, the operation of the display device 100 in a black display modewill be described. In the black display mode, the liquid crystal layer 6is controlled to exhibit a transmitting state by being supplied with avoltage. In such a situation, externally incoming light is transmittedthrough the substrate 8, color filter layer 7 and liquid crystal layer6. The light that has been transmitted through the liquid crystal layer6 is retro-reflected by the reflective electrode 11 on the micro cornercube array 10. That is to say, before entering the eyes of the observerwho is watching an image on the display, the incoming light is refractedby the substrate 8 and liquid crystal layer 6, retro-reflected from themicro corner cube array 10 and then refracted by the liquid crystallayer 6 and substrate 8 again. Thus, only the light that has come fromthe vicinity of the observer's eyes goes out of this display device 100toward the observer. In this case, if the vicinity of the observer'seyes is too narrow a region to allow any light source to be presentthere (i.e., if that region is smaller in area than the observer'spupil), then a good black display is realized.

As described above, the light that has been incident onto the microcorner cube array 10 is reflected back in the direction that isprecisely opposite to the direction in which the incoming light hastraveled. However, the retro-reflected outgoing light ray is slightlyshifted horizontally (or translated) from the incoming light ray. Theshift is approximately equal to the size (or pitch) of each unit elementof the micro corner cube array 10. Accordingly, if the size L1 of eachunit element of the micro corner cube array 10 is greater than the sizeL2 of each picture element region as shown in FIG. 10A, then the colorof a color filter that the incoming light ray passes (e.g., green (G) inthe example illustrated in FIG. 10A) is different from that of a colorfilter that the outgoing light ray passes (e.g., blue (B) in the exampleillustrated in FIG. 10A), thus causing a color mixture unintentionally.

On the other hand, if the size L1 of each unit element of the microcorner cube array 10 is smaller than the size L2 of each picture elementregion as shown in FIG. 10B, then the color of a color filter that theincoming light ray passes (e.g., green (G) in the example illustrated inFIG. 10B) is the same as that of the color filter that the outgoinglight ray passes, thus causing no color mixture. Accordingly, to displayan image in desired colors, the size L1 of each unit element of themicro corner cube array 10 needs to be smaller than the size L2 of eachpicture element region. In the micro corner cube array 10 for use inthis preferred embodiment, the size of each unit element (e.g., about 10μm) is defined sufficiently smaller than a normal size of a pictureelement region (e.g., several tens μm) as already described for thefirst and second preferred embodiments. Thus, an image can beappropriately displayed in desired colors.

Next, the retro-reflectivity of a light ray that has been incidentstraight onto a corner cube consisting of three rectangular isoscelestriangular planes will be compared with that of a light ray that hasbeen incident straight onto a corner cube consisting of three squareplanes. It should be noted that when a light ray is incident onto apoint of a corner cube, the light ray is reflected back in the oppositedirection from a point which is symmetric to the point of incidence withrespect to the center of the corner cube. This is a necessary andsufficient condition for a corner cube.

FIGS. 11A through 11C illustrate a situation where each corner cubeconsists of three rectangular isosceles triangular planes. Specifically,FIG. 11A illustrates corner cubes, while FIGS. 11B and 11C illustrate anarray of corner cubes. In the situation where each corner cube consistsof three rectangular isosceles triangular planes, when those threeconstituent planes are projected onto a reference plane, the projectionhas an equilateral triangular shape as shown in FIG. 11C. In that case,if a light ray is incident onto a point of the corner cube near one ofthe vertices of the equilateral triangle, then the light ray is notretro-reflected because inside the corner cube, there is no point thatis symmetric to the point of incidence with respect to the center of thecorner cube. Accordingly, the retro-reflectivity is at most about 66%.

On the other hand, FIGS. 12A through 12C illustrate a situation whereeach corner cube consists of three square planes. Specifically, FIG. 12Aillustrates a corner cube, while FIGS. 12B and 12C illustrate an arrayof corner cubes. In the situation where each corner cube consists ofthree square planes, when those three constituent planes are projectedonto a reference plane, the projection has a rectangular hexagonal shapeas shown in FIG. 12C. In that case, no matter where the light ray isincident, each and every point of incidence has a point symmetric withrespect to the center of the corner cube. Accordingly, a light ray thathas been incident onto any point of the rectangular hexagon is alwaysretro-reflected. Thus, it can be seen that to get an incoming light rayretro-reflected as intended, each of the micro corner cubes in an arraypreferably consists of square planes and the projection of theconstituent planes on a reference plane preferably has a rectangularhexagonal shape.

In the micro corner cube array for use in this preferred embodiment,each unit element thereof includes three substantially square planes,defined by {100} planes of cubic single crystals, as already describedfor the first and second preferred embodiments. Thus, the micro cornercube array can retro-reflect the incoming light just as intended. Thatis to say, in the black display mode, the observer senses no unwantedlight. As a result, an appropriate dark display is realized and thecontrast ratio increases.

The present inventors measured the reflectivities and contrast ratios ofthe reflective type liquid crystal display device 100 including themicro corner cube array 10 of this preferred embodiment and acomparative reflective type liquid crystal display device 800 includingno micro corner cube array as shown in FIG. 13. Specifically, thereflective type liquid crystal display device 800 as a comparativeexample uses a scattering reflector 11 instead of the micro corner cubearray 10. Accordingly, while the liquid crystal layer 6 thereof is inthe transmitting state, light that has been emitted from a light sourcedistant from the observer may also be reflected toward the observer.Thus, this display device 800 further includes a polarizer 13 and aphase plate 14 in front of the substrate 8 to absorb the unwantedreflected light and realize a good display of color black. It should benoted that the other members (e.g., the liquid crystal layer 6 and colorfilter layer 7) of the display device 800 are the same as thecounterparts of the display device 100.

The reflectivity and contrast ratio of each of these display devices 100and 800 were measured by using an apparatus such as that shown in FIG.14. As shown in FIG. 14, this apparatus is constructed so that diffusedlight is emitted from an integrating sphere 15 toward a sample 16 (i.e.,the display device 100 or 800) and that the light reflected from thesample 16 is received at a photodetector 17 disposed in front of thesample 16. The results are shown in the following Table 1:

TABLE 1 Reflectivity (%) in white display Contrast ratio Display device100 28 23 Display device 800 15 25

As can be seen from these results, the liquid crystal display device 100of this preferred embodiment, which uses the micro corner cube array 10instead of the polarizer 13 or phase plate 14, can display a brightimage at a relatively high contrast ratio and with good visibility.

EMBODIMENT 6

Hereinafter, a sixth specific preferred embodiment of the presentinvention will be described. The sixth preferred embodiment relates toan organic electroluminescence (EL) display device (i.e., aself-light-emitting display device) that includes a micro corner cubearray according to the first or second preferred embodiment of thepresent invention described above.

FIG. 15 illustrates a configuration for an organic EL display deviceaccording to this preferred embodiment. As shown in FIG. 15, the organicEL display device 200 includes upper and lower substrates 30 and 34 andan organic EL layer 42 provided between the substrates 30 and 34. Theupper substrate 30 may be made of a transparent material such as glassor a polymer film. The lower substrate 34 is disposed so as to face theupper substrate 30. The organic EL layer 42 is made up of a plurality ofthin films including hole injected layer, hole transporting layer, lightemitting layer, electron transporting layer and electron injected layer.Also, a cathode (or transparent electrode) 32, made of a transparentconductive material such as indium tin oxide (ITO), is interposedbetween the organic EL layer 42 and the upper substrate 30. Furthermore,an anode 40 is provided between the organic EL layer 42 and the lowersubstrate 34. The anode 40 may be made of an aluminum film with athickness of about 30 nm, for example. When the anode 40 is made of sucha thin film, the anode 40 can transmit light.

A micro corner cube array reflector 36, obtained by the method describedfor the first or second preferred embodiment, is provided on one surfaceof the lower substrate 34 that is opposed to the organic EL layer 42.Although not shown in FIG. 15, the surface of this micro corner cubearray reflector 36 is covered with a reflective film of aluminum, forexample. Thus, when a light ray is incident onto this micro corner cubearray reflector 36, the light ray is reflected back to its source. Therugged surface of the micro corner cube array reflector 36, covered withthe reflective film, is flattened by a transparent flattening member 38.And the anode 40 is located on this flattened surface.

In this organic EL display device 200, when a predetermined voltage isapplied to the organic EL layer 42 between the cathode 32 and anode 40,electrons and holes, which have moved from the cathode 32 and anode 40,respectively, are recombined with each other in the organic EL layer 42,thereby causing the organic EL layer 42 to emit luminescence andconducting a display operation as intended. This organic EL layer 42 maybe made of any of various known materials by any of various knowntechniques.

In this organic EL display device 200, when the organic EL layer 42 isemitting no luminescence (i.e., while the display device 200 is in ablack display mode), an incoming light ray, which has been externallyincident from the vicinity of the observer (e.g., from an electric lampor the sun) onto this display device 200, is reflected by the microcorner cube array reflector 36 back to the external light source anddoes not reach the observer's eyes. Thus, the unwanted back reflectionof the external light can be prevented and color black can be displayedjust as intended.

On the other hand, when the organic EL layer 42 is emitting luminescence(i.e., while the display device 200 is in a white display mode), notonly a portion of the luminescence that has been emitted from theorganic EL layer 42 toward the observer but also another portion of theluminescence going toward the lower substrate 34 also reach theobserver's eyes. This is because the latter portion of the luminescenceis retro-reflected by the micro corner cube array reflector 36 towardthe observer. Accordingly, the luminescence that has been emitted fromthe organic EL layer 42 can be utilized more efficiently and color whitewith high lightness can be displayed just as intended. In addition, evenin the white display mode, the unwanted back reflection of the externallight, which has been emitted from some light source around theobserver, is also avoidable.

In this organic EL display device 200, the size of each unit element inthe micro corner cube array reflector 36 is also preferably smaller thanthe size of each picture element region as in the liquid crystal displaydevice of the fifth preferred embodiment. The organic EL layer 42 ofthis organic EL display device 200 includes a plurality of lightemitting regions that emit luminescence in the three primary colors ofred (R), green (G) and blue (B) as shown in FIG. 15. That is to say,these light emitting regions correspond to the picture element regionsin the display device 100 of the fifth preferred embodiment describedabove. If the size of each unit element of the micro corner cube arrayreflector 36 is smaller than that of each picture element region, then alight ray, which has been emitted from a light emitting region in apredetermined color and then reflected from the micro corner cube arrayreflector 36, does not pass through an adjacent light emitting region ina different color. Thus, no color mixture is created and decrease inbrightness or chromaticity is avoidable.

EMBODIMENT 7

Hereinafter, a seventh specific preferred embodiment of the presentinvention will be described. The seventh preferred embodiment relates toa method of making a micro corner cube array that has an optical axistilted away from a normal to the reference plane of a base material. Adisplay device including such a micro corner cube array is disclosed inU.S. Pat. No. 5,182,663 issued to Jones of Raychem Corporation.

First, a GaAs substrate, having a surface tilted away from the (111)Bplanes of GaAs crystals by about 5 degrees, is prepared. In thispreferred embodiment, a GaAs substrate is used as in the first preferredembodiment. Alternatively, a cubic single crystalline substrate made ofany other material (e.g., a substrate consisting of germanium singlecrystals according to the second preferred embodiment) may also be usedas long as the substrate has a surface that is tilted away from the{111} planes of the crystals by a predetermined angle (e.g., from about0 degrees to about 10 degrees).

Next, the GaAs substrate prepared is subjected to the mirror polishing,photoresist pattern definition and wet etching process steps as in thefirst preferred embodiment, thereby forming multiple corner cubes, eachbeing made up of three {100} planes of the crystals that are opposedsubstantially perpendicularly to each other (e.g., (100), (010) and(001) planes), on the surface of the substrate. In this manner, a cornercube array is obtained. In this preferred embodiment, however, the GaAssubstrate has a surface that is tilted away from {111}B planes of thecrystals by about 5 degrees unlike the first preferred embodiment.Accordingly, the angle defined by each of the three planes of eachcorner cube with the reference plane of the substrate (i.e., theoriginal surface of the substrate yet to be etched) is different fromthat of the first preferred embodiment. Also, each of the three planesof a corner cube formed in this manner may have a rectangular shape.

When such a single crystalline substrate having a surface that is tiltedaway from the {111}B planes of the crystals is used, the aspect ratio ofeach masking element of the etching mask layer (i.e., pattern of theetching mask layer) is preferably changed with the tilt angle. This isbecause in this preferred embodiment, when a corner cube obtained isviewed from over the substrate, the corner cube does not have theperfectly rectangular hexagonal shape unlike the first preferredembodiment (see FIG. 2D, for example), but may be slightly elongatedeither vertically or horizontally depending on the tilt angle. Also,where the corner cubes are formed by the method of this preferredembodiment, the median point of each masking element does not have tomatch with its associated honeycomb lattice point completely butapproximately. That is to say, the median points of some maskingelements may be slightly shifted from their associated honeycomb latticepoints.

If the corner cube array that has been formed in this manner on thesurface of the GaAs substrate is coated with a reflective film asalready described for the first or second preferred embodiment, then thecorner cube array may be used as a retroreflector.

In this preferred embodiment, however, the pattern of the micro cornercube array that has been formed on the surface of the GaAs substrate istransferred onto a resin 20 as shown in FIG. 16, thereby making a microcorner cube array of the resin 20. More specifically, first, anelectroformed mold 18 is made from the GaAs substrate by a knowntechnique. Next, this electroformed mold 18 is attached onto a roller19. And then the electroformed mold 18 is rotated by the roller 19 andpressed against the resin 20, thereby transferring the pattern of themicro corner cube array onto the resin 20.

FIG. 16 also shows the direction 21 in which the roller 19 is rotatedand the direction 22 in which the resin 20 is transported. In FIG. 16,the line A-B is parallel to the direction 22 in which the resin 20 istransported. As the roller 19 is rotated in the direction 21 and as theresin 20 is transported in the direction 22, the rugged surface of theelectroformed mold 18 is pressed against the resin 20 and then the resin20 is stripped from the electroformed mold 18. The direction in whichthe resin 20 is stripped from the mold 18 is also parallel to the lineA-B.

FIGS. 17A and 17B illustrate the micro corner cube array that has beenformed (or transferred) onto the surface of the resin 20 by theabove-described technique. In FIGS. 17A and 17B, the line A-B indicatesthe same direction as that shown in FIG. 16. As described above, theresin 20 onto which the pattern of the micro corner cube array has beentransferred is stripped along the line A-B. In this preferredembodiment, the resin 20 is stripped in such a manner that the line A-B,along which the resin 20 is stripped, and a normal 23 to one 24 of thethree square planes of each corner cube are present within the sameplane. That is to say, as can be seen from FIG. 17B, the projection ofthe normal 23 onto a plane representing the surface of the resin 20 isparallel to the line A-B along which the resin 20 is stripped. In thatcase, the resin 20 can be stripped and removed more easily compared to asituation where the resin 20 is stripped non-parallelly to theprojection of the normal 23 onto the plane representing the resinsurface.

To strip the resin 20 in this manner, the pattern of the electroformedmold 18 needs to be transferred onto the resin 20 in such a manner thatthe line A-B (i.e., the direction 22) along which the resin 20 istransported as shown in FIG. 16 and a normal to one of the three planesof each micro corner cube on the electroformed mold 18 are alwayspresent within the same plane (e.g., on the paper of FIG. 16). Thispositional relationship is easily realizable by adjusting the directionthat the electroformed mold 18 being attached onto the surface of theroller 19 faces.

Also, when the electroformed mold 18, which has been made from the GaAssubstrate having a surface tilted from the (111)B plane by about 5degrees, is used as in this preferred embodiment, the resin 20 can bestripped and removed more easily compared to using an electroformed moldmade from the GaAs substrate having a surface parallel to the (111)Bplane as in the first preferred embodiment.

The micro corner cube array obtained in this manner has its optical axistilted away from a normal to the reference plane of the substrate. Thus,as for a retroreflector obtained from this micro corner cube array, arange where the incoming light ray can be retro-reflected from theretroreflector appropriately (which will be herein referred to as an“incoming light ray range”) is defined around the tilted optical axis.In this case, any light ray going from this incoming light ray rangetoward the retroreflector is retro-reflected appropriately, whereas alight ray going from outside of this incoming light ray range toward theretroreflector may be retro-reflected inappropriately. Accordingly,where a light source is located above a display device, if theretroreflector is disposed such that its optical axis is tilted from anormal to the panel plane of the display device toward the light source(i.e., upward), then the retroreflector can retro-reflect any light raycoming from the light source. As a result, the display device candisplay color black even more satisfactorily.

In the preferred embodiment described above, the substrate is supposedto have a surface that is tilted from the {111} planes of the crystalsby about 5 degrees. However, the tilt angle of the substrate surface isnot limited thereto but may be equal to or greater than about 0 degreesand equal to or smaller than about 10 degrees. The angle defined betweenthe surface of the substrate and the {111} planes is equal to the angledefined by the optical axis of each micro corner cube to be formed witha normal to the substrate surface. In this case, the optical axis ofeach micro corner cube is defined as a line that is equally distant fromthe three perpendicularly opposed planes that make up the corner cube.Generally speaking, a micro corner cube exhibits its bestretro-reflectivity along its optical axis. In other words, if theoptical axis of a micro corner cube is directed toward a light source,then any light ray that has come from the light source onto the microcorner cube is reflected from the micro corner cube right back to thelight source. As a result, color black can be displayed just asintended. However, if the optical axis of a micro corner cube is overlytilted from a normal to the substrate surface, then theretro-reflectivity of the micro corner cube is not so good for anobserver who is watching the display approximately along the normal tothe substrate surface. Then, in the black display mode, the observer mayalso sense an unnecessary light ray that has been emitted from a lightsource distant from his or her eyes (or pupils) and then reflected fromthe micro corner cube back to the observer. In that case, color blackmight be displayed inappropriately. In view of these considerations, thepresent inventors carried out experiments to define an appropriate tiltangle range for the optical axis of a corner cube. The results of theexperiments revealed that the optical axis of a corner cube preferablyhas a tilt angle of equal to or greater than about 0 degrees and equalto or smaller than about 10 degrees. Accordingly, to make a corner cubehaving such an optical axis, a substrate, having a surface that istilted from the {111} planes by about 0 degrees to about 10 degrees, ispreferably used.

Also, as shown in FIG. 18, when a single crystalline substrate, having asurface S0 that is tilted away from the {111} planes of crystals by anangle θ, is used, the intersection L3 between the {111} planes and thesurface S0 preferably crosses a predetermined cleaved facet of thesingle crystalline substrate at right angles. If the substrate is madeof GaAs single crystals, the predetermined cleaved facet is a (01-1)plane. In other words, a plane that includes a normal L4 to the {111}planes and a normal L5 to the substrate surface S0 is preferablyparallel to the predetermined cleaved surface of the substrate. In thatcase, the respective planes that make up a corner cube can have theirsymmetry increased. For example, as shown in FIG. 17B, each corner cubemay have a shape which is vertically symmetrical about a line. Also,when an array of corner cubes obtained in this manner is used as a die,the material can be removed from the die easily.

EMBODIMENT 8

As described above, the fifth preferred embodiment of the presentinvention relates to a reflective type liquid crystal display device 100including a retroreflector that has been made from a micro corner cubearray 10 as shown in FIG. 9. Hereinafter, a reflective type liquidcrystal display device of that type will be described in further detailas an eighth specific preferred embodiment of the present invention.

The present inventors carried out an intensive research to improve thedisplay performance of a reflective type display device including ascattering type liquid crystal layer as a light modulating layer and aretroreflector. As a result, the present inventors discovered that ifthe scattering type liquid crystal layer is disposed adjacent to thereflective planes of the retroreflector, the desired black and whitedisplay modes are both realizable by utilizing the transmitting andscattering states of the scattering type liquid crystal layer,respectively. In such an arrangement, the unwanted absorption of lightinto the electrode 12 or the flattening member 95 as observed in theconventional reflective type liquid crystal display device 900 shown inFIG. 40 is avoidable. Thus, the incoming light can be utilized much moreefficiently. Furthermore, since no flattening member 95 is provided forthe reflective type display device 100 of the present invention, theprocess load necessary for fabricating the display device can belightened and the fabrication cost can be reduced.

It should be noted that the light modulating layer (e.g., the scatteringtype liquid crystal layer in this embodiment) and the reflective planesof the retroreflector are herein regarded as being “adjacent to eachother” not only when the reflective planes of the retroreflector and thelight modulating layer are actually in contact with each other but alsowhen some additional member such as an alignment film is interposedbetween the reflective planes of the retroreflector and the lightmodulating layer. In the latter case, however, the lower surface of thelight modulating layer needs to be defined as a surface that is alignedwith the reflective planes of the retroreflector.

Also, this arrangement is applicable for use in not just the displaydevice using the scattering type liquid crystal layer as a lightmodulating layer but also in a reflective type display device of any ofvarious other types including a light modulating layer that can switchbetween the transmitting and scattering states.

It should be noted, however, that when a corner cube reflector is usedas the retroreflector having the reflective planes that are adjacent tothe scattering type liquid crystal layer, an incoming light ray, whichhas been transmitted through the scattering type liquid crystal layer 6in the transmitting state, sometimes goes a relatively long distance inthe in-plane directions (i.e., x and y directions) of the liquid crystallayer 6 as shown in FIG. 34 while being reflected back to its source. Insuch a situation, to get the incoming light reflected back to its sourcejust as intended, the scattering type liquid crystal layer in thetransmitting state preferably scatter or reflect as small a quantity oflight as possible in the in-plane (x and y) directions of the liquidcrystal layer. This is because if there is any scattering factor in theoptical path of the incoming light passing through the scattering typeliquid crystal layer 6, then the retro-reflectivity decreases and thedesired dark display is not realizable. A problem like this was notobserved in the conventional reflective type liquid crystal displaydevice 900 shown in FIG. 40.

Thus, in the reflective type display device of the present invention,the scattering type liquid crystal layer (or light modulating layer) inthe transmitting state preferably exhibits good continuity in theorientation vectors of liquid crystal molecules not only in thethickness (z) direction thereof but also in the in-plane (x and y)directions thereof. In other words, according to the present invention,the liquid crystal layer preferably preserves good orientationcontinuity. It should be noted that if the liquid crystal molecules areoriented continuously in one of these predetermined directions, thetransmittance of a light ray going in the predetermined direction withrespect to the air is typically about 70% or more.

Alternatively, the scattering type liquid crystal layer in thetransmitting state preferably exhibits good continuity in refractiveindices with respect to not only the light going in the thickness (z)direction but also the light going in the in-plane (x and y) directions.It should be noted that if the liquid crystal layer exhibits goodrefractive index continuity in one of these predetermined directions,the transmittance of a light ray going in the predetermined directionwith respect to the air is typically about 70% or more.

As such a scattering type liquid crystal layer, a polymer-dispersedliquid crystal layer, consisting essentially of a liquid crystal phaseand a polymer phase, may be used. The polymer phase is formed bypolymerizing a monomer having a liquid crystal skeleton. In thepolymer-dispersed liquid crystal layer, the refractive indices of theliquid crystal and polymers phases are preferably substantially equal to(i.e., matched with) each other in any of the thickness and in-planedirections (i.e., x, y and z directions) thereof.

Alternatively, a scattering type liquid crystal layer, which createsliquid crystal domains having sizes of about 100 nm to about 20,000 nmin its scattering state and which scatters the incoming light byutilizing the difference in refractive index between the liquid crystaldomains, may also be used. A scattering type liquid crystal layer likethis may be a liquid crystal gel layer obtained by mixing a liquidcrystal material with a gelling agent. While the scattering type liquidcrystal layer is in the transmitting state on the other hand, the liquidcrystal molecules thereof are aligned substantially uniformly over theentire liquid crystal layer. As used herein, the “liquid crystal domain”refers to a zone defined in the liquid crystal layer by the boundaries(or disclinations) in which the liquid crystal molecules are orienteddiscontinuously.

A scattering type liquid crystal layer like this is particularlyeffectively applicable for use in a display device in which the cornercubes are arranged at a relatively large pitch (or have a relativelygreat size) in the in-plane (x and y) directions of the liquid crystallayer. If the corner cubes are arranged at a relatively large pitch,then the light ray going in the in-plane (x and y) directions will havea longer optical path. In such a case, the degree of scattering of thelight ray going in the in-plane (x and y) directions is changeablegreatly depending on whether or not the liquid crystal moleculespreserve good orientation or refractive index continuity in thosedirections. This dependency is especially noticeable when the opticalpath length of a light ray going in the in-plane (x and y) directions isgreater than that of a light ray going in the thickness (z) direction.The optical path length of the light ray going in the z direction may bedefined by the maximum thickness of the liquid crystal layer, forexample, which may be about 1 μm to about 50 μm.

If the liquid crystal molecules can preserve good orientation orrefractive index continuity in the in-plane (x and y) directions asdescribed above, the unwanted scattering of the light can be minimizedeven when the corner cubes are arranged at a relatively large pitch.Thus, the retro-reflectivity can be increased, the desired dark displayis realized, and an image can be displayed at a high contrast ratio.

The present inventors discovered and confirmed via experiments that toincrease the retro-reflectivity, a scattering type liquid crystal layerlike this is very effectively applicable for use in a display device inwhich the corner cubes are arranged at a pitch of about 1 μm or more.However, if the corner cubes have too large a size, then the unwantedlight may much more likely reach the observer's eyes in the blackdisplay mode. For that reason, the arrangement pitch of the corner cubesis preferably about 1,000 μm or less.

Accordingly, by applying such a scattering type liquid crystal layer, inwhich the liquid crystal molecules preserve good orientation orrefractive index continuity in the in-plane (x and y) directions, to adisplay device including corner cubes that are arranged at a pitch ofabout 1 μm to about 1,000 μm, the display device can display an imagejust as intended.

Hereinafter, a reflective type liquid crystal display device accordingto the eighth preferred embodiment of the present invention will bedescribed in further detail.

FIG. 34 is a cross-sectional view illustrating a configuration for apart of the reflective type liquid crystal display device 100 shown inFIG. 9 to a larger scale. As shown in FIGS. 9 and 34, this liquidcrystal display device 100 includes a pair of substrates 8 and 9 and ascattering type liquid crystal layer 6, which is provided as a lightmodulating layer between the substrates 8 and 9. The substrate 8 islocated closer to the observer, while the substrate 9 is disposed so asto face the substrate 8. Both of these substrates 8 and 9 may be made ofa transparent material and may be glass plates or polymer films.

On one surface of the substrate 8 that is opposed to the liquid crystallayer 6, a color filter layer 7, including R, G and B color filters, anda transparent electrode 12 are stacked in this order. On the other hand,on one surface of the substrate 9 that is opposed to the liquid crystallayer 6, a micro corner cube array 10 is provided. This micro cornercube array 10 is covered with a reflective electrode 11, which is madeof a material with a high surface reflectivity (e.g., silver oraluminum) and has a substantially uniform thickness. The reflectiveelectrode 11 may be formed so as to come into tight contact with therugged surface of the micro corner cube array 10 by depositing silver toa thickness of about 200 nm by an evaporation process, for example. Thereflective electrode 11 serves not only as reflective planes forreflecting the incoming light back to its source but also as anelectrode for applying a voltage to the liquid crystal layer 6. In thisreflective type liquid crystal display device 100, the micro corner cubearray 10 and the reflective electrode 11 together make up a corner cubereflector 28 functioning as a retroreflector.

In this reflective type liquid crystal display device 100, thescattering type liquid crystal layer 6 is adjacent to the reflectiveplane 11 a of the corner cube reflector 28 (i.e., the surfaces of thereflective electrode 11). It should be noted, however, that thescattering type liquid crystal layer 6 does not have to be in contactwith the reflective plane 11 a. Optionally, an alignment film (notshown) for providing orientation control capability for the scatteringtype liquid crystal layer 6 may be additionally formed on the reflectiveplane 11 a.

The liquid crystal display device 100 having such a configurationdisplays an image thereon by controlling the light modulating states ofthe liquid crystal layer 6 on a pixel-by-pixel basis with a voltageapplied from the transparent electrode 12 and reflective electrode 11 tothe liquid crystal layer 6. As shown in FIG. 34, the reflectiveelectrode 11 may be driven by a known active component 25 (such as athin-film transistor) which is electrically connected to the reflectiveelectrode 11 by way of a contact hole 26. Alternatively, the reflectiveelectrode 11 may also be driven by any other driving means.

As already described for the fifth preferred embodiment, the scatteringtype liquid crystal layer 6 may be made of a polymer-dispersed liquidcrystal material. However, the material of the liquid crystal layer 6 isnot limited thereto. Examples of other preferred materials for thescattering type liquid crystal layer 6 will be described in furtherdetail later.

Hereinafter, it will be described with reference to FIGS. 35A through35E how the reflective type liquid crystal display device 100 may befabricated.

First, as shown in FIG. 35A, an active component 25 and other membersare formed on a substrate 9.

Next, as shown in FIG. 35B, a corner cube array 10 is formed over thesubstrate 9. The corner cube array 10 may be formed by depositing aresin film to a thickness of about 10 μm over the substrate 9 and thenpressing a preformed corner cube array mold onto the resin film, forexample.

As already described for the first through fourth preferred embodiments,if a substrate consisting of cubic single crystals is etchedanisotropically by utilizing the difference in etch rate between twogroups of crystal planes thereof, then an array of corner cubes, eachbeing made up of three substantially square, substantiallyperpendicularly opposed planes S1, S2 and S3 as shown in FIG. 39, can beobtained. Also, if the mold is made from the corner cube array obtainedin this manner and then pressed against the resin film that has beendeposited over the substrate 9, then a corner cube array 10 can be madeof the resin material in the same shape as that shown in FIG. 39. As canbe seen from FIG. 34, the corner cube array 10 has a rugged surface.Accordingly, the thickness of the liquid crystal layer 6 is changeablefrom one position to another, thus possibly affecting theelectro-optical properties of the liquid crystal layer 6 too much. Thus,it is not preferable that the size of the corner cubes is too large. Forthat reason, the arrangement pitch P of the corner cubes is preferablyabout 50 μm or less. In the corner cube array 10 having such a shape asthat shown in FIG. 39, the level difference between the highest- andlowest-level points of the corner cube array 10 may be defined by thearrangement pitch P of the corner cubes. Accordingly, the preferablesize of the corner cubes is defined in this preferred embodiment by thearrangement pitch P of the corner cubes.

Next, as shown in FIG. 35C, the corner cube array 10 is subjected tosome processing such as a plasma ashing process by using a predeterminedresist mask (not shown), thereby forming contact holes 26, which will beused to establish electrical connection with the active components 25,at predetermined positions of the corner cube array 10.

Subsequently, as shown in FIG. 35D, a conductive material such as silveris vertically deposited by an evaporation process, for example, to athickness of about 200 nm over the surface of the corner cube array 10so that the contact holes 26 thereof are also filled with the conductivematerial. Then, the conductive material deposited is patterned, therebyforming a reflective electrode 11, which is electrically connected tothe active components 25 and other members, over the corner cube array10. In this manner, a corner cube reflector 28, consisting of the cornercube array 10 and the reflective electrode 11 and functioning as aretroreflector, is obtained.

Thereafter, as shown in FIG. 35E, a color filter layer 7 and atransparent electrode 12 are formed by known techniques on a transparentsubstrate 8 to obtain a counter substrate. Then, the substrate 9including the corner cube reflector 28 thereon and the counter substrateare bonded together with a predetermined gap left between them. Finally,a liquid crystal material is injected into the gap and the injectionholes are sealed to form a scattering type liquid crystal layer 6. Inthis manner, a reflective type liquid crystal display device including aretroreflector is completed.

Hereinafter, it will be described with reference to FIG. 36 how thereflective type liquid crystal display device 100 operates. First, itswhite display mode operation will be described. In the white displaymode, the liquid crystal layer 6 is controlled to exhibit a scatteringstate as shown in the right half of FIG. 36. While the liquid crystallayer 6 is in the scattering state, externally incoming light isscattered by the liquid crystal layer 6 after having been transmittedthrough the substrate 8, color filter layer 7 and transparent electrode12. In this case, portion of the incoming light, which has beenbackscattered by the liquid crystal layer 6, returns to the observer. Inaddition, in the display device 100 of this preferred embodiment,another portion of the incoming light that has been transmitted straightthrough the liquid crystal layer 6 and still another portion of theincoming light that has been forward scattered by the liquid crystallayer 6 are also reflected by the reflective electrode 11 on the microcorner cube array 10. The reflected light is scattered again by theliquid crystal layer 6 in the scattering state while passing through theliquid crystal layer 6. As a result, a portion of the scattered lightreturns to the observer. In this manner, in the white display mode, notonly the backscattered light but also a portion of the forward scatteredlight return to the observer, thus realizing a display of a highlybright image.

Next, the operation of the display device 100 in a black display modewill be described. In the black display mode, the liquid crystal layer 6is controlled to exhibit a transmitting state by being supplied with avoltage as shown in the left half of FIG. 36. In such a situation, theexternally incoming light is transmitted through the substrate 8, colorfilter layer 7, transparent electrode 12 and liquid crystal layer 6. Thelight that has been transmitted through the liquid crystal layer 6 isretro-reflected by the reflective electrode 11 on the micro corner cubearray 10. That is to say, before entering the eyes of the observer whois watching an image on the display, the incoming light is refracted bythe substrate 8 and liquid crystal layer 6, retro-reflected from themicro corner cube array 10 and then refracted by the liquid crystallayer 6 and substrate 8 again. Thus, only the light that has come fromthe vicinity of the observer's eyes goes out of this display device 100toward the observer. In this case, if the vicinity of the observer'seyes is too narrow a region to allow any light source to be presentthere (i.e., if that region is smaller in area than the observer'spupil), then color black can be displayed just as intended.

As shown in FIG. 36, while the liquid crystal layer 6 is in thetransmitting state (i.e., while the device 100 is in the black displaymode), at least portion of the light that is passing through the liquidcrystal layer 6 can go in any of the thickness (z) and in-plane (x andy) directions of the liquid crystal layer 6. Accordingly, toretro-reflect this light appropriately, the liquid crystal layer 6 inthe transmitting state preferably scatters as small a fraction of theincoming light as possible in each of the x, y and z directions.Particularly when the arrangement pitch (or the size) of the cornercubes 10 is relatively large (e.g., about 1 μm or more) compared to thethickness of the liquid crystal layer 6 (e.g., about 1 μm to about 50μm), the transparency of the liquid crystal layer 6 to the light goingin the in-plane (i.e., x and y) directions thereof is a critical factorto be considered.

In this preferred embodiment, the scattering type liquid crystal layer 6is made of a polymer-dispersed liquid crystal material as describedabove. In this case, the liquid crystal layer 6 includes a polymer phasenot responding to an electric field applied (i.e., a matrix portion ofthe liquid crystal layer 6) and a liquid crystal phase made up of liquidcrystal molecules responding to the applied electric field. If thepolymer phase has a size large enough to scatter light (e.g., about 100nm to about 20,000 nm), then it depends heavily on a difference inrefractive index between the liquid crystal and polymer phases whetheror not the light is scattered. It should be noted that the “size” of thepolymer phase herein means the size of the polymer phase located betweenthe liquid crystal phases and typically refers to a size correspondingto an average distance between the liquid crystal phases.

Accordingly, to minimize such scattering of the light due to thedifference in refractive index between the liquid crystal and polymerphases, the polymer and liquid crystal phases of the liquid crystallayer in the transmitting state preferably have approximately equalrefractive indices. That is to say, the refractive indices of these twophases preferably substantially match with each other. Particularly whenthe incoming light goes a relatively long distance not only in thethickness (z) direction but also in the in-plane (x and y) directions asin this preferred embodiment, the two types of phases of the scatteringtype liquid crystal layer in the transmitting state should haveapproximately equal refractive indices with respect to the light goingin the thickness direction or to the light going in the x and ydirections. If the polymer and liquid crystal phases have approximatelyequal refractive indices (i.e., if the refractive index does not changesteeply in the interface between these two phases), then the liquidcrystal layer can exhibit good refractive index continuity in each ofthe x, y and z directions (i.e., either in the thickness direction or inthe in-plane directions). In that ideal situation, the liquid crystallayer can retro-reflect the incoming light appropriately.

Next, suppose a non-ideal situation where the two types of phases of aliquid crystal layer in the transmitting state have approximately equalrefractive indices only in the thickness (z) direction and substantiallydifferent ones in the in-plane (x and y) directions. Examples of suchscattering type liquid crystal layers include: a polymer-dispersedliquid crystal layer made up of a polymer phase, obtained bypolymerizing a monomer with no refractive index anisotropy, and a liquidcrystal phase; and a scattering type liquid crystal layer obtained bypermeating a matrix of a material having an isotropic refractive indexwith a liquid crystal material.

When the liquid crystal layer is made of any of these materials, anetwork is formed by a transparent material having no refractive indexanisotropy (e.g., the polymer phase) in the liquid crystal layer.Normally, an appropriate combination of materials is selected so thatthe refractive index n_(p) of the transparent material is equal to theordinary index n_(oLC) of the liquid crystal material. However, theordinary index n_(oLC) of a liquid crystal material is usually smallerthan the extraordinary index n_(eLC) thereof (i.e., n_(oLC)<n_(eLC)).Accordingly, the extraordinary index of the liquid crystal material isnot equal to the refractive index of the transparent material. In thatcase, such a scattering type liquid crystal layer exhibits a transparentstate on the application of a voltage thereto. In the transparent state,the incoming light may enter the liquid crystal layer approximatelyvertically from over the liquid crystal layer. Since the refractiveindex n_(p) of the transparent material is equal to the ordinary indexn_(oLC) of the liquid crystal material in the thickness (z) direction,the incoming light goes straight through the liquid crystal layerwithout being scattered.

Thereafter, the incoming light is reflected by a first reflective planeof the corner cube reflector and then translated horizontally inside theliquid crystal layer as shown in FIG. 36. In this case, the refractiveindex n_(p) of the transparent material is not equal to theextraordinary index n_(eLC) of the liquid crystal material in thein-plane (x and y) directions of the liquid crystal layer. Thus, due tothe difference in refractive index, the light is scattered and cannot beretro-reflected appropriately. Consequently, the desired dark display isnot realized.

In view of these considerations, while the liquid crystal layer is inthe transmitting state, the difference in refractive index between thepolymer phase (first phase) and the liquid crystal phase (second phase)is preferably as small as possible with respect to the light going inthe thickness direction or the light going in the in-plane directions.More specifically, the difference is preferably within about 5%. Inother words, the ratio of the refractive indices of these two phases ispreferably about 0.95 to about 1.05. If the scattering type liquidcrystal layer in the transmitting state can exhibit good refractiveindex continuity with respect to both the light going in the thicknessdirection and the light going in the in-plane directions, the unwantedscattering of the light can be minimized and an image can be displayedjust as intended.

The technique of controlling the refractive indices described above isapplied to a scattering type liquid crystal layer including a polymerphase that causes the incoming light to be scattered while the liquidcrystal layer is in the transmitting state. However, the liquid crystaldisplay device of this preferred embodiment may also include ascattering type liquid crystal layer which includes no scatteringfactors such as the polymer phase but in which the incoming light stillmay be scattered due to a difference in refractive index between theliquid crystal domains to be created therein as a result of a voltagecontrol. It should be noted that even when some dispersion phases (e.g.,particles) of too small a size to scatter the incoming light are presentin a liquid crystal layer, that liquid crystal layer is also hereinregarded as “including no scattering factors”.

A scattering type liquid crystal layer of that type may exhibit thescattering state when a plurality of liquid crystal domains of apredetermined size (e.g., about 100 nm to about 20,000 nm) is formedtherein on the application of a voltage thereto. On the other hand, whenthe liquid crystal molecules of the liquid crystal layer are oriented toform no liquid crystal domains therein, the liquid crystal layer mayexhibit a transmitting state. Examples of such scattering type liquidcrystal layers include: a liquid crystal gel; a micelle dispersed liquidcrystal layer in which a micelle having a diameter of about 5 nm toabout 100 nm has been formed and dispersed in a liquid crystal material;a liquid crystal suspension layer (or superfine particle dispersedliquid crystal layer) in which solid particles having a diameter ofabout 5 nm to about 100 nm are dispersed in a liquid crystal material;an amorphous nematic liquid crystal layer; and a cholesteric-nematicphase change type liquid crystal layer.

To achieve the desired transmitting state, such a scattering type liquidcrystal layer in the transmitting state preferably exhibits goodcontinuity in the orientation vectors of liquid crystal molecules notonly in the thickness (z) direction thereof but also in the in-plane (xand y) directions thereof. That is to say, the liquid crystal layershould preferably preserve good orientation continuity in each of the x,y and z directions. In other words, the liquid crystal molecules arepreferably aligned substantially uniformly in the entire scattering typeliquid crystal layer in the transmitting state. If the liquid crystallayer in the transmitting state can also maintain good orientationcontinuity in the in-plane (x and y) directions, then the light going inthose directions is not scattered. As a result, that light can also beretro-reflected just as intended.

It should be noted that such a scattering type liquid crystal layer mayinclude some dispersion phases (such as the micelle described above)having a predetermined size (e.g., about 100 nm or less), which is toosmall to scatter the light, and may also have a plurality of liquidcrystal domains formed through the misalignment of the liquid crystalmolecules due to the presence of those dispersion phases. Thosedispersion phases do not contribute to scattering of the incoming light.Thus, the refractive index of those dispersion phases does not have tobe equal to that of the liquid crystal phase unlike the polymer phasedescribed above.

As described above, the scattering type liquid crystal layer of theliquid crystal display device of this preferred embodiment may be madeof any of various liquid crystal materials as long as the liquid crystallayer can switch between the scattering and transmitting states. In thetransmitting state, the scattering type liquid crystal layer preferablyexhibits good continuity in the orientation vectors of the liquidcrystal molecules both in the thickness direction and in the in-planedirections that are perpendicular to the thickness direction.Alternatively, the scattering type liquid crystal layer in thetransmitting state may exhibit good continuity in refractive index withrespect to the light going in the thickness direction and the lightgoing in the in-plane directions.

Hereinafter, specific examples of the reflective type liquid crystaldisplay device of this eighth preferred embodiment will be described. Inthe following illustrative examples, various liquid crystal materialsfor the scattering type liquid crystal layer will be quoted.

EXAMPLE 1

A first specific example of the reflective type liquid crystal displaydevice of the eighth preferred embodiment will be described withreference to FIG. 37. In the first specific example, a UV-cured,polymer-dispersed liquid crystal layer is used as the scattering typeliquid crystal layer.

The polymer-dispersed liquid crystal layer of the first example isobtained by preparing a mixture of a low-molecular-weight liquid crystalcomposition and an unpolymerized prepolymer having a liquid crystalskeleton in a miscible state, injecting the mixture into the gap betweenthe substrates and then polymerizing the prepolymer. Any type ofprepolymer may be used as long as the prepolymer has a liquid crystalskeleton and can be aligned with the liquid crystal composition. In thisexample, a mixture of a UV-curing prepolymer and a liquid crystalcomposition is photo-cured by being exposed to an active ray such as anultraviolet ray, and the cured mixture (i.e., a UV-cured liquid crystalmaterial) is used as the polymer-dispersed liquid crystal material. Whensuch a UV-cured liquid crystal material is used as the polymer-dispersedliquid crystal material, the polymerizable liquid crystal material maybe polymerized while maintaining its original liquid crystal orientationbefore the polymerization and without being heated unnecessarily.

The prepolymer-liquid crystal mixture of this specific example may beobtained by adding a small amount of polymerization initiator (producedby Ciba-Geigy Corporation, for example) to a mixture of a UV-curingmaterial (e.g., Mix C produced by DIC Corporation) and a liquid crystalmaterial having negative dielectric anisotropy (e.g., ZLI-4318 producedby Merck & Co., Inc.) that have been mixed at a weight ratio of about10:90, for example.

In this specific example, a vertical alignment film that can align theliquid crystal molecules substantially vertically to the film isprovided over each of the electrodes sandwiching the liquid crystallayer. Also, the liquid crystal molecules in this liquid crystal layerhave negative dielectric anisotropy. Accordingly, while no voltage isbeing applied to the liquid crystal layer, the liquid crystal moleculesexisting over the reflective planes of the corner cube reflector or overthe counter substrate have their orientation state controlled such thattheir major axes are substantially aligned with the thickness directionof the liquid crystal layer. It should be noted that when “the majoraxes of the liquid crystal molecules are substantially aligned with thethickness direction of the liquid crystal layer”, the angle definedbetween the major axes of the liquid crystal molecules and the thicknessdirection of the liquid crystal layer is herein supposed to be less thanabout 45 degrees.

To display an image at a high contrast ratio, the liquid crystal layerof this liquid crystal display device should be as transparent aspossible while no voltage is being applied to the liquid crystal layer.For that purpose, the UV-curing material Mix C needs to have itsrefractive index matched with that of the liquid crystal material. Therefractive indices of these materials may be matched by adjusting thedifference Δn in refractive index between these materials and/or byselecting an appropriate combination of materials. For example, beforepolymerized, Mix C has an extraordinary index of about 1.66 and anordinary index of about 1.51 at room temperature. On the other hand,after having been polymerized, Mix C has an extraordinary index of about1.64 and an ordinary index of about 1.52 at room temperature.Accordingly, the liquid crystal material to be combined with Mix Cpreferably has extraordinary and ordinary refractive indices fallingwithin the ranges defined by those of Mix C before and after thepolymerization. That is to say, the liquid crystal material preferablyhas an extraordinary index of about 1.64 to about 1.66 and an ordinaryindex of about 1.51 to about 1.52.

The refractive indices of these materials may also be matched with eachother by controlling the refractive index of the polymer with anadditive mixed with the prepolymer. By appropriately defining the typeand the amount of the additive, the refractive index of the polymer maybe substantially equalized with that of the liquid crystal materialused. Thus, it is possible to use a liquid crystal material having anarbitrary refractive index for the liquid crystal layer whilemaintaining sufficient transparency.

In this specific example, a liquid crystal material having negativedielectric anisotropy is used for the liquid crystal layer and verticalalignment films are used. However, the liquid crystal material and thetype of the alignment films are not limited, thereto. For example, aliquid crystal material having positive dielectric anisotropy andhorizontal alignment films may also be used. Alternatively, a hybridorientation state or a bent orientation state may also be created byselecting an appropriate combination of materials.

However, if the liquid crystal molecules are aligned horizontally, thenthe liquid crystal molecules of the liquid crystal layer in thetransparent state have their orientation state affected or disturbed bythe ruggedness of the corner cube array, thus creating disclinations andscattering the incoming light unintentionally. As a result, color blackcannot be displayed as intended. The present inventors discovered andconfirmed via experiments that if a liquid crystal material havingnegative dielectric anisotropy and vertical alignment films are used incombination, then the liquid crystal layer can exhibit good continuityin the orientation vectors of its liquid crystal molecules and can alsoshow a high degree of transparency with no disclinations, thusdisplaying color black just as intended. For that reason, a liquidcrystal material having negative dielectric anisotropy and verticalalignment films are used in this specific example.

Next, it will be described exactly how the reflective type liquidcrystal display device, including such a polymer-dispersed liquidcrystal layer as the scattering type liquid crystal layer, conducts adisplay operation. First, a white display mode operation thereof will bedescribed. In the white display mode, while a voltage is being appliedto the polymer-dispersed liquid crystal layer 6, the liquid crystalmolecules of the liquid crystal phase have their orientation statechanged in response to an electric field applied in the thicknessdirection of the cell (or the liquid crystal layer) while the polymerphase is polymerized and does not respond to the electric field as shownin the right half of FIG. 37. Thus, in such a state, the refractiveindices of the liquid crystal and polymer phases are not equal to eachother both in the thickness direction and in-plane directions of theliquid crystal layer. As a result, the liquid crystal layer 6 exhibitsthe scattering state. Accordingly, portion of the light that has enteredthe liquid crystal layer 6 is forward scattered by the liquid crystallayer 6, reflected from the retroreflector 28 and then scattered againby the liquid crystal layer 6 in the scattering state. Consequently, notonly the backscattered light but also a lot of other light return to theobserver.

That is to say, the reflective type liquid crystal display device ofthis specific example utilizes not only the inefficient backscatteredlight but also the forward scattered light that has passed through theliquid crystal layer 6, thereby displaying a bright image. In thisspecific example, no flattening member or transparent electrode ispresent on the substrate 9. Thus, no light is absorbed into any of thesemembers in vain and a very bright image can be displayed.

Next, a black display mode operation thereof will be described. In theblack display mode, while no voltage is being applied to the liquidcrystal layer 6, the liquid crystal molecules and the polymer phase,which retains the orientation of the liquid crystal molecules, havetheir refractive indices substantially matched with each other in eachof the x, y and z directions. Accordingly, the light that has enteredthe liquid crystal layer 6 is not scattered no matter which directionthe light has taken, and is retro-reflected appropriately. Consequently,color black can be displayed just as intended.

EXAMPLE 2

Next, a second specific example of the reflective type liquid crystaldisplay device, including a liquid crystal gel layer as the scatteringtype liquid crystal layer, will be described with reference to FIG. 38.

In this specific example, a liquid crystal gel layer is obtained byadding a hydrogen-bonding low-molecular-weight gelling agent,represented by the following chemical formula (1), to a nematic liquidcrystal material:

More specifically, a liquid crystal gel layer in the scattering state isobtained by adding about 0.5 mol/L of the gelling agent to a nematicliquid crystal material (e.g., TL-204 produced by Merck Ltd).

This gelling agent forms a random hydrogen-bonding network in the liquidcrystal solvent, thus accelerating the creation of liquid crystaldomains and causing the liquid crystal layer to exhibit a scatteringstate while no voltage is being applied thereto. It should be noted thatthe liquid crystal domains to be created may have a size of about 100 nmto about 20,000 nm and can scatter the incoming light. Also, in responseto an electric field applied, the liquid crystal molecules of thisliquid crystal gel layer are substantially aligned and the liquidcrystal gel layer goes transparent. In this case, the liquid crystal gellayer exhibits good continuity in the orientation vectors of its liquidcrystal molecules both in the thickness direction and in the in-planedirections thereof. Accordingly, the incoming light can be appropriatelyretro-reflected from this liquid crystal gel layer.

When an image was displayed by a reflective type liquid crystal displaydevice including such a liquid crystal gel layer, the image displayedshowed an excellent viewing angle characteristic. This is because in aliquid crystal gel layer in the transparent state, the refractiveindices can be matched more perfectly than in a polymer-dispersed liquidcrystal layer.

EXAMPLE 3

Next, a third specific example of the reflective type liquid crystaldisplay device, including a liquid crystal emulsion layer as thescattering type liquid crystal layer that exhibits the scattering statewhen liquid crystal domains are created, will be described withreference to FIG. 38.

The liquid crystal emulsion layer of the third specific example isobtained by mixing a liquid crystal material with a liquid, which is notmiscible with the liquid crystal material, and molecules behaving as asurfactant. In that case, emulsification occurs in the mixture, therebyforming a micro-emulsion, in which a liquid phase is dispersed in theliquid crystal material, as a dispersion phase.

By precisely controlling the droplet diameter of this micro-emulsion,the micro-emulsion can be used as a scattering medium that responds toan electric field created. Thus, an inverted micelle W/LC is formed byadding about 5 wt % of an aqueous solution of a didodecyl ammonium saltto a liquid crystal material (e.g., TL-204 produced by Merck & Co.,Inc.) and used as a micro-emulsion in the scattering state for thereflective type liquid crystal display device.

The micelle colloid to be formed preferably has a size of about 100 nmor less so as not to constitute a scattering factor by itself when anelectric field is applied thereto. However, if its size is too small,then the micelle colloid will have no influence on the orientation stateof the liquid crystal molecules. For that reason, the micelle colloidpreferably has a size of at least about 5 nm. By controlling theconcentration and the size of the micelle colloid, the liquid crystalemulsion layer in the transparent state can increase its transparencyand an image can be displayed at an increased contrast ratio.

In this specific example, the droplets dispersed in the emulsion (i.e.,the dispersion phases) preferably have a size of about 5 nm to about 100nm. The reason is as follows. First, the upper limit of about 100 nm isdefined because while the liquid crystal emulsion layer is in thetransparent state, the droplets should have a size sufficiently smallerthan the wavelength of the incoming light (i.e., about 300 nm to about800 nm) so as not to scatter the incoming light. On the other hand, thelower limit of about 5 nm is defined because of the following reason.Specifically, to have a significant effect on the orientation state ofthe liquid crystal molecules and realize the desired random orientationand scattering state, the dispersed particles should be sufficientlygreater than the liquid crystal molecules having approximate dimensionsof 2 nm by 0.5 nm. Thus, a minimum required size for controlling theorientation state of the liquid crystal molecules is herein defined atabout 5 nm.

EXAMPLE 4

Next, a fourth specific example of the reflective type liquid crystaldisplay device, including a micelle dispersed liquid crystal colloidlayer as the scattering type liquid crystal layer, will be described.

The micro-emulsion of the third specific example described above is notparticularly limited to a mixture of water and a liquid crystalmaterial. In order to increase the reliability of the liquid crystalmixture, a mixture of fluorocarbon and a liquid crystal material israther preferred. Thus, as another exemplary micro-emulsion, ascattering medium in which a micelle of fluorocarbon is dispersed in aliquid crystal material is made by mixing the liquid crystal materialwith an organic compound containing a small amount of perfluoro base asrepresented by the following chemical formula (2):

As in the third specific example, the micelle colloid to be formed inthis specific example also preferably has a size of about 100 nm or lessso as not to constitute a scattering factor when an electric field isapplied thereto. However, if its size is too small, the micelle colloidwill have no influence on the orientation state of the liquid crystalmolecules. Accordingly, the micelle colloid preferably has a size of atleast about 5 nm. When the scattering type liquid crystal layer of thisspecific example was used, the liquid crystal layer exhibited increasedcharge retention ability and the liquid crystal display device can bedriven by active components just as intended.

EXAMPLE 5

Next, a fifth specific example of the reflective type liquid crystaldisplay device, including a liquid crystal suspension layer as thescattering type liquid crystal layer, will be described.

The liquid crystal suspension layer of this specific example can beobtained by dispersing, as a colloid, fine particles of titanium oxide,which have been subjected to a surface treatment with butanol, stearicacid or other suitable substance, in a liquid crystal material. Thesurface treatment is conducted to align the liquid crystal moleculesvertically to the surface of the titanium oxide fine particles. Thosefine particles accelerate the creation of liquid crystal domains. Oncethose liquid crystal domains have been created, the liquid crystalsuspension layer exhibits a scattering state. More specifically, fineparticles of titanium oxide having an average diameter of about 50 nm,of which the surface had been treated with butanol, were uniformlydispersed at a concentration of about 5 wt % in a liquid crystalmaterial (e.g., TL-204 produced by Merck & Co., Inc.), thereby obtaininga liquid crystal suspension layer.

To disperse those fine particles even more stably, a polymer materialwas added to the mixture. Because of the same reason as that alreadydescribed for the third specific example, the superfine particlespreferably have a size of about 5 nm to about 100 nm in this specificexample also. However, by selecting an appropriate combination of typeand size for the additive superfine particles, the liquid crystal layerin the transmitting state showed increased transparency and the liquidcrystal display device realized a highly reliable display.

EXAMPLE 6

Next, a sixth specific example of the reflective type liquid crystaldisplay device, including an amorphous nematic liquid crystal layer asthe scattering type liquid crystal layer, will be described.

In the specific examples described above, an impurity that controls theorientation state of liquid crystal molecules is mixed with a liquidcrystal material to randomize the orientation of the liquid crystalmolecules. However, if the alignment film used is not subjected to anyalignment treatment such as a rubbing treatment or if the liquid crystalmolecules are in a cholesteric orientation state in which it isdifficult to preserve good continuity in the orientation vectors of theliquid crystal molecules, no impurity needs to be mixed with the liquidcrystal material on purpose. This is because in such a situation, liquidcrystal domains can be created and a scattering state is realizedwithout adding any impurity thereto.

In this specific example, no alignment films were provided for the pairof substrates that sandwiched the liquid crystal layer, and a displayoperation was conducted by using a nematic liquid crystal material(e.g., TL-204 produced by Merck & Co., Inc.) exhibiting positivedielectric anisotropy. As a result, an image could be displayed at ahigh contrast ratio, the liquid crystal layer showed increased chargeretention ability and the liquid crystal display device could be drivenjust as intended by active components. In this specific example, amaterial exhibiting positive dielectric anisotropy was used for theliquid crystal layer. Alternatively, a liquid crystal material havingnegative dielectric anisotropy may also be used instead.

EXAMPLE 7

Next, a seventh specific example of the reflective type liquid crystaldisplay device, including a cholestericnematic phase change type liquidcrystal layer as the scattering type liquid crystal layer, will bedescribed.

It is known that a cholesteric liquid crystal material, having a pitchapproximately equal to an infrared wavelength, exhibits a planarorientation state and a transmitting state when no electric field isapplied thereto but exhibits a focal conic orientation state, in whichtheir screw axes are randomly oriented from one domain to another, and ascattering state when an electric field is applied thereto. Thisscattering medium can be produced without using any alignment film,which is advantageous for cost reduction. In addition, since itsscattering state is memorized, a device that uses this medium can bedriven at lower power dissipation. Thus, in this specific example, anappropriate chiral agent (e.g., S1011 produced by Merck & Co., Inc.) wasmixed with a liquid crystal material (e.g., TL-204 produced by Merck &Co., Inc.) so that the liquid crystal molecules had a chiral pitch ofabout 1.0 μm, thereby forming a cholesteric-nematic phase change typeliquid crystal layer. Even when the cholestericnematic phase change typeliquid crystal layer obtained in this manner was used, an image couldalso be displayed at a high contrast ratio.

As described above, if the scattering type liquid crystal layer as alight modulating layer is made of an appropriate material, then theunwanted scattering of the incoming light can be minimized both in thethickness direction of the liquid crystal layer and in the in-planedirections thereof while the liquid crystal layer is in the transmittingstate. As a result, color black can be displayed just as intended and animage can be displayed at a high contrast ratio. It should be noted thatas long as the liquid crystal layer 6 can switch between the scatteringand transmitting states, the liquid crystal layer 6 does not have to bethe scattering type liquid crystal layer but may be any other type oflight modulating layer. Specifically, examples of other usable liquidcrystal layers include: a cholesteric liquid crystal layer, which canswitch between transmitting and reflecting states and to which diffusionproperties are imparted by controlling the sizes of liquid crystaldomains; a polymer-dispersed liquid crystal layer with a holographicfunction, which switches between transmitting and reflecting states andto which diffusion properties are imparted by being exposed to diffusingradiation; and a polymer-dispersed liquid crystal layer, which switchesbetween absorbing and scattering modes (e.g., a polymer-dispersed liquidcrystal layer to which a dye has been added).

A corner cube array for the reflective type liquid crystal displaydevice described above may be made by using a single crystallinesubstrate consisting of cubic crystals (i.e., cubic single crystallinesubstrate). The cubic single crystalline substrate may be made of acompound semiconductor having a sphalerite structure or a materialhaving a diamond structure. More specifically, a cubic singlecrystalline substrate, having its surface disposed substantiallyparallelly to {111} planes of the crystals, is prepared and has thatsurface patterned by being subjected to an anisotropic etching process.

In this method, the surface of the substrate is patterned by ananisotropic etching process so that the etch rate of one crystal planeis different from that of another. For example, if the substrate is madeup of GaAs crystals having a sphalerite structure, the etch rate of the{111}B planes of the crystals (i.e., the {111} planes of arsenic) isrelatively high, while the etch rate of the {100} planes (i.e., crystalplanes including (100), (010) and (001) planes) thereof is relativelylow. Accordingly, the etching process advances anisotropically in such amanner as to leave the {100} planes of the crystals. As a result,concavo-convex portions are defined on the surface of the substrate bymultiple unit elements, each being made up of the {100} planes of thecrystals. Each of those unit elements that have been formed in thismanner has three perpendicularly opposed planes (e.g., (100), (010) and(001) planes), thus forming a corner cube.

In a corner cube array formed by such a method, the three reflectiveplanes of each corner cube are matched with the {100} crystallographicplanes of a cubic crystal and exhibit very high shape precision. Also,the three reflective planes that make up each corner cube have goodplanarity, and each corner or edge, at which two or three of themintersect with each other, has sufficient sharpness. Furthermore, thecorner cube array has a stereoscopic shape in which multiple cornercubes are arranged in a regularly pattern. In this array, the respectivevertices of the corner cubes are located at substantially the same level(or within substantially the same plane). Thus, a corner cube array likethis can be used effectively as a retroreflector for reflecting anincoming light ray back to its source.

Also, the size of each unit element (i.e., each corner cube) of thearray formed by the method of the present invention may be several tensμm or less by controlling the feature size of a photoresist pattern (ormask) used in the etching process. Accordingly, a corner cube array of avery small size, which is suitably applicable for use as aretroreflector for a liquid crystal display device, for example, can beobtained.

In a micro corner cube array obtained in this manner, each unit elementthereof has three substantially square planes that are defined by {100}planes of cubic single crystals, and can reflect an incoming light rayback to its source. Accordingly, in a black display mode, a liquidcrystal display device, including such a micro corner cube array as acorner cube reflector, realizes desired dark display without reflectingunwanted light back to the observer. As a result, the contrast ratioalso increases.

According to the present invention, a micro corner cube array is made byanisotropically etching the {111} planes of a cubic single crystallinesubstrate and forming a plurality of unit elements each being made up ofcrystallographic planes that have been etched at a relatively low etchrate (e.g., {100} planes). Thus, a micro corner cube array, consistingof very small unit elements of a size (e.g., about several tens μm)smaller than that of a picture element region of the display device andyet showing very high shape precision, can be made through relativelysimple process steps.

A display device including such a micro corner cube array can displaycolor black just as intended without using any polarizer, and candisplay a bright image at a high contrast ratio, a high color purity andhighly visibility.

Also, in a reflective type display device according to the presentinvention, which includes a retroreflector and a light modulating layerthat can switch between scattering and transmitting states, the lightmodulating layer is disposed adjacently to the reflective planes of theretroreflector. Thus, the reflective type display device can displaycolor white at a high lightness and at a high contrast ratio.

While the present invention has been described with respect to preferredembodiments thereof, it will be apparent to those skilled in the artthat the disclosed invention may be modified in numerous ways and mayassume many embodiments other than those specifically described above.Accordingly, it is intended by the appended claims to cover allmodifications of the invention that fall within the true spirit andscope of the invention.

1. A method of making a micro corner cube array, the method comprising:preparing a substrate, at least a surface portion of which consists ofcubic single crystals and which has a surface that is substantiallyparallel to {111} planes of the crystals; and etching the surface of thesubstrate anisotropically, thereby forming a plurality of unit elementsfor the micro corner cube array on the surface of the substrate, eachsaid unit element being made up of a number of crystal planes that havebeen formed at a lower etch rate than the {111} planes of the crystals,wherein the unit element comprises three substantially square planesthat are oriented substantially perpendicularly to each other.
 2. Themethod of claim 1, wherein the etching step comprises the step offorming {100} planes of the crystals at the lower etch rate than the{111} planes thereof.
 3. The method of claim 2, wherein the etching stepcomprises the step of forming the unit elements so that each said unitelement is made up of three {100} planes that are opposed substantiallyperpendicularly to each other.
 4. The method of claim 3, wherein atleast the surface portion of the substrate prepared in the preparingstep is made of a compound semiconductor having a sphalerite structure.5. The method of claim 4, wherein the compound semiconductor is galliumarsenide and the substrate has a surface that is substantially parallelto {111}B planes formed by arsenic atoms.
 6. The method of claim 3,wherein at least the surface portion of the substrate prepared in thepreparing step is made of a material having a diamond structure.
 7. Themethod of claim 6, wherein at least the surface portion of the substrateconsists of germanium single crystals.
 8. The method of claim 1, whereinthe etching step comprises the step of etching the surface of thesubstrate anisotropically so that a ratio of the etch rate of the {111}planes to the lower etch rate of the crystal planes is greater than1.73.
 9. The method of claim 1, further comprising, between thepreparing and etching, covering the surface of the substrate with anetching mask layer, the etching mask layer including at least onemasking element and at least one opening that have been arranged to forma predetermined pattern.
 10. The method of claim 9, wherein the etchingstep comprises the step of forming the unit elements for the microcorner cube array so that the size of each said unit element iscontrolled in accordance with the pattern of the etching mask layerdefined in the covering step.
 11. The method of claim 9, wherein thecovering step comprises the step of defining the etching mask layer thatincludes a plurality of masking elements, each said masking elementhaving a median point that is substantially located at a honeycomblattice point.
 12. The method of claim 11, wherein the masking elementsare spaced apart from each other.
 13. The method of claim 9, wherein themasking element has a planar shape defined by at least three sides thatare respectively parallel to (100), (010) and (001) planes of thecrystals.
 14. The method of claim 13, wherein the masking element has atriangular planar shape defined by the three sides.
 15. The method ofclaim 9, wherein the masking element has a planar shape defined by atleast three sides that are respectively parallel to (11-1), (1-11) and(−111) planes of the crystals.
 16. The method of claim 15, wherein themasking element has a triangular planar shape defined by the threesides.
 17. The method of claim 9, wherein the masking element has aplanar shape that is symmetrical about a three-fold rotation axis. 18.The method of claim 17, wherein the masking element has a hexagonal,nonagonal or dodecagonal planar shape.
 19. The method of claim 9,wherein the etching mask layer includes a plurality of openings, eachsaid opening having a median point that is substantially located at ahoneycomb lattice point.
 20. The method of claim 9, wherein the at leastone masking element accounts for more than 50% of the total area of theetching mask layer.
 21. The method of claim 9, wherein the etching stepcomprises the step of stopping etching the surface of the substrate whena contact area between the surface of the substrate and the maskingelement is substantially minimized.
 22. The method of claim 1, whereinthe etching step comprises the step of subjecting the surface of thesubstrate to a wet etching process.
 23. The method of claim 22, whereinthe etching step further comprises the step of subjecting the surface ofthe substrate to a dry etching process at least once.
 24. The method ofclaim 1, further comprising the step of transferring the shape of theunit elements, which have been formed on the surface of the substrate,to a resin material.
 25. The method of claim 1, wherein the surface ofthe substrate prepared in the step defines an angle of greater than 0degrees and equal to or smaller than 10 degrees with the {111} planes ofthe crystals.
 26. The method of claim 25, wherein an intersectionbetween the surface of the substrate and the {111} planes of thecrystals is substantially perpendicular to a cleaved facet of thesubstrate.
 27. A method of making an array of micro corner cubes, eachsaid corner cube being defined by predetermined crystal planes of acrystal having a prescribed structure, the method comprising: preparinga substrate, at least a surface portion of which consists of thecrystals having the prescribed structure; and etching the substrateanisotropically in a manner so that the predetermined crystal planes areetched at a lower rate than {111} planes of the crystal, therebyexposing the predetermined crystal planes intentionally.
 28. A method ofmaking a micro corner cube array, the method comprising: preparing asubstrate, at least a surface portion of which comprises cubic singlecrystals and which has a surface that is substantially parallel to {111}planes of the crystals; and etching the surface of the substrate atleast anisotropically, thereby forming a plurality of unit elements forthe micro corner cube array on the surface of the substrate, each saidunit element comprising a number of crystal planes that have been formedat a lower etch rate than the {111} planes of the crystals, wherein theunit element comprises three substantially square planes that areoriented substantially perpendicularly to each other.
 29. The method ofclaim 28, wherein said etching comprises forming {100} planes of thecrystals at the lower etch rate than the {111} planes thereof.
 30. Themethod of claim 28, wherein said etching comprises forming the unitelements so that each said unit element is made up of three {100} planesthat are opposed substantially perpendicularly to each other.
 31. Themethod of claim 28, further comprising using the micro corner cube arrayin a display apparatus.
 32. A method of making an array of micro cornercubes, each said corner cube comprising substantially square crystalplanes of a crystal having a prescribed structure, the methodcomprising: preparing a substrate, at least a surface portion of whichcomprises the crystals having the prescribed structure; and etching thesubstrate in a manner so that said crystal planes are etched at a lowerrate than {111} planes of the crystal, thereby exposing thesubstantially square crystal planes.